C2Y adds unsigned versions of the abs functions (see C2Y draft N3467 and
proposal N3349).
Tested for x86_64.
Signed-off-by: Lenard Mollenkopf <glibc@lenardmollenkopf.de>
If the input buffer exceeds the stack auxiliary buffer, qsort will
malloc a temporary one to call mergesort. Since C++ standard does
allow the callback comparison function to throw [1], the glibc
implementation can potentially leak memory.
The fixes uses a pthread_cleanup_combined_push and
pthread_cleanup_combined_pop, so it can work with and without
exception enables. The qsort code path that calls malloc now
requires some extra setup and a call to __pthread_cleanup_push
anmd __pthread_cleanup_pop (which should be ok since they just
setup some buffer state).
Checked on x86_64-linux-gnu.
[1] https://timsong-cpp.github.io/cppwp/n4950/alg.c.library#4
Reviewed-by: DJ Delorie <dj@redhat.com>
This helps with statically interposing getenv.
Updates commit 7a61e7f557
("stdlib: Make getenv thread-safe in more cases").
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
For the originally failing application (userhelper from usermode),
it is not actually necessary to call realloc on the environ
pointer. Yes, there will be a memory leak because the application
assigns a heap-allocated pointer to environ that it never frees,
but this leak was always there: the old realloc-based setenv had
a hidden internal variable, last_environ, that was used in a similar
way to __environ_array_list. The application is not impacted by
the leak anyway because the relevant operations do not happen in
a loop.
The change here just uses a separte heap allocation and points
environ to that. This means that if an application calls
free (environ) and restores the environ pointer to the value
at process start, and does not modify the environment further,
nothing bad happens.
This change should not invalidate any previous testing that went into
the original getenv thread safety change, commit 7a61e7f557
("stdlib: Make getenv thread-safe in more cases").
The new test cases are modeled in part on the env -i use case from
bug 32588 (with !DO_MALLOC && !DO_EARLY_SETENV), and the previous
stdlib/tst-setenv-malloc test. The DO_MALLOC && !DO_EARLY_SETENV
case in the new test should approximate what userhelper from the
usermode package does.
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Some applications set environ to a heap-allocated pointer, call
setenv (expecting it to call realloc), free environ, and then
restore the original environ pointer. This breaks after
commit 7a61e7f557 ("stdlib: Make
getenv thread-safe in more cases") because after the setenv call,
the environ pointer does not point to the start of a heap allocation.
Instead, setenv creates a separate allocation and changes environ
to point into that. This means that the free call in the application
results in heap corruption.
The interim approach was more compatible with other libcs because
it does not assume that the incoming environ pointer is allocated
as if by malloc (if it was written by the application). However,
it seems to be more important to stay compatible with previous
glibc version: assume the incoming pointer is heap allocated,
and preserve this property after setenv calls.
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
As the test comment explains, this test is not quite valid, but
preserving the exact sequences helps distributions to port to
newer glibc versions. We can remove this test if we ever switch
to a different implementation.
Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>
The initstate{,_r} interfaces are documented in BSD as needing an aligned
array of 32-bit values, but neither POSIX nor glibc's own documentation
require it to be aligned. glibc's documentation says it "should" be a power
of 2, but not must.
Use memcpy to read and write to `state` to handle such an unaligned
argument.
Co-authored-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Reviewed-by: Florian Weimer <fweimer@redhat.com>
A plain indirect function call does not work on POWER because
success and failure are signaled through a flag register, and
not via the usual Linux negative return value convention.
This has potential security impact, in two ways: the return value
could be out of bounds (EAGAIN is 11 on powerpc6le), and no
random bytes have been written despite the non-error return value.
Fixes commit 461cab1de7 ("linux: Add
support for getrandom vDSO").
Reported-by: Ján Stanček <jstancek@redhat.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Since internal tests don't have access to internal symbols in libm,
exclude them for internal tests. Also make tst-strtod5 and tst-strtod5i
depend on $(libm) to support older versions of GCC which can't inline
copysign family functions. This fixes BZ #32414.
Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Sunil K Pandey <skpgkp2@gmail.com>
Async-signal-safety is preserved, too. In fact, getenv is fully
reentrant and can be called from the malloc call in setenv
(if a replacement malloc uses getenv during its initialization).
This is relatively easy to implement because even before this change,
setenv, unsetenv, clearenv, putenv do not deallocate the environment
strings themselves as they are removed from the environment.
The main changes are:
* Use release stores for environment array updates, following
the usual pattern for safely publishing immutable data
(in this case, the environment strings).
* Do not deallocate the environment array. Instead, keep older
versions around and adopt an exponential resizing policy. This
results in an amortized constant space leak per active environment
variable, but there already is such a leak for the variable itself
(and that is even length-dependent, and includes no-longer used
values).
* Add a seqlock-like mechanism to retry getenv if a concurrent
unsetenv is observed. Without that, it is possible that
getenv returns NULL for a variable that is never unset. This
is visible on some AArch64 implementations with the newly
added stdlib/tst-getenv-unsetenv test case. The mechanism
is not a pure seqlock because it tolerates one write from
unsetenv. This avoids the need for a second copy of the
environ array that getenv can read from a signal handler
that happens to interrupt an unsetenv call.
No manual updates are included with this patch because environ
usage with execve, posix_spawn, system is still not thread-safe
relative unsetenv. The new process may end up with an environment
that misses entries that were never unset. This is the same issue
described above for getenv.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Linux 6.11 has getrandom() in vDSO. It operates on a thread-local opaque
state allocated with mmap using flags specified by the vDSO.
Multiple states are allocated at once, as many as fit into a page, and
these are held in an array of available states to be doled out to each
thread upon first use, and recycled when a thread terminates. As these
states run low, more are allocated.
To make this procedure async-signal-safe, a simple guard is used in the
LSB of the opaque state address, falling back to the syscall if there's
reentrancy contention.
Also, _Fork() is handled by blocking signals on opaque state allocation
(so _Fork() always sees a consistent state even if it interrupts a
getrandom() call) and by iterating over the thread stack cache on
reclaim_stack. Each opaque state will be in the free states list
(grnd_alloc.states) or allocated to a running thread.
The cancellation is handled by always using GRND_NONBLOCK flags while
calling the vDSO, and falling back to the cancellable syscall if the
kernel returns EAGAIN (would block). Since getrandom is not defined by
POSIX and cancellation is supported as an extension, the cancellation is
handled as 'may occur' instead of 'shall occur' [1], meaning that if
vDSO does not block (the expected behavior) getrandom will not act as a
cancellation entrypoint. It avoids a pthread_testcancel call on the fast
path (different than 'shall occur' functions, like sem_wait()).
It is currently enabled for x86_64, which is available in Linux 6.11,
and aarch64, powerpc32, powerpc64, loongarch64, and s390x, which are
available in Linux 6.12.
Link: https://pubs.opengroup.org/onlinepubs/9799919799/nframe.html [1]
Co-developed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Tested-by: Jason A. Donenfeld <Jason@zx2c4.com> # x86_64
Tested-by: Adhemerval Zanella <adhemerval.zanella@linaro.org> # x86_64, aarch64
Tested-by: Xi Ruoyao <xry111@xry111.site> # x86_64, aarch64, loongarch64
Tested-by: Stefan Liebler <stli@linux.ibm.com> # s390x
As for exit, also allows concurrent quick_exit to avoid race
conditions when it is called concurrently. Since it uses the same
internal function as exit, the __exit_lock lock is moved to
__run_exit_handlers. It also solved a potential concurrent when
calling exit and quick_exit concurrently.
The test case 'expected' is expanded to a value larger than the
minimum required by C/POSIX (32 entries) so at_quick_exit() will
require libc to allocate a new block. This makes the test mre likely to
trigger concurrent issues (through free() at __run_exit_handlers)
if quick_exit() interacts with the at_quick_exit list concurrently.
This is also the latest interpretation of the Austin Ticket [1].
Checked on x86_64-linux-gnu.
[1] https://austingroupbugs.net/view.php?id=1845
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Even if C/POSIX standard states that exit is not formally thread-unsafe,
calling it more than once is UB. The glibc already supports
it for the single-thread, and both elf/nodelete2.c and tst-rseq-disable.c
call exit from a DSO destructor (which is called by _dl_fini, registered
at program startup with __cxa_atexit).
However, there are still race issues when it is called more than once
concurrently by multiple threads. A recent Rust PR triggered this
issue [1], which resulted in an Austin Group ask for clarification [2].
Besides it, there is a discussion to make concurrent calling not UB [3],
wtih a defined semantic where any remaining callers block until the first
call to exit has finished (reentrant calls, leaving through longjmp, and
exceptions are still undefined).
For glibc, at least reentrant calls are required to be supported to avoid
changing the current behaviour. This requires locking using a recursive
lock, where any exit called by atexit() handlers resumes at the point of
the current handler (thus avoiding calling the current handle multiple
times).
Checked on x86_64-linux-gnu and aarch64-linux-gnu.
[1] https://github.com/rust-lang/rust/issues/126600
[2] https://austingroupbugs.net/view.php?id=1845
[3] https://www.openwall.com/lists/libc-coord/2024/07/24/4
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Complete the internal renaming from "C2X" and related names in GCC by
renaming *-c2x and *-gnu2x tests to *-c23 and *-gnu23.
Tested for x86_64, and with build-many-glibcs.py for powerpc64le.
WG14 decided to use the name C23 as the informal name of the next
revision of the C standard (notwithstanding the publication date in
2024). Update references to C2X in glibc to use the C23 name.
This is intended to update everything *except* where it involves
renaming files (the changes involving renaming tests are intended to
be done separately). In the case of the _ISOC2X_SOURCE feature test
macro - the only user-visible interface involved - support for that
macro is kept for backwards compatibility, while adding
_ISOC23_SOURCE.
Tested for x86_64.
The following patch uses the GCC 14 __builtin_stdc_* builtins in stdbit.h
for the type-generic macros, so that when compiled with GCC 14 or later,
it supports not just 8/16/32/64-bit unsigned integers, but also 128-bit
(if target supports them) and unsigned _BitInt (any supported precision).
And so that the macros don't expand arguments multiple times and can be
evaluated in constant expressions.
The new testcase is gcc's gcc/testsuite/gcc.dg/builtin-stdc-bit-1.c
adjusted to test stdbit.h and the type-generic macros in there instead
of the builtins and adjusted to use glibc test framework rather than
gcc style tests with __builtin_abort ().
Signed-off-by: Jakub Jelinek <jakub@redhat.com>
Reviewed-by: Joseph Myers <josmyers@redhat.com>
The mergesort removal from qsort implementation (commit 03bf8357e8)
had the side-effect of making sorting nonstable. Although neither
POSIX nor C standard specify that qsort should be stable, it seems
that it has become an instance of Hyrum's law where multiple programs
expect it.
Also, the resulting introsort implementation is not faster than
the previous mergesort (which makes the change even less appealing).
This patch restores the previous mergesort implementation, with the
exception of machinery that checks the resulting allocation against
the _SC_PHYS_PAGES (it only adds complexity and the heuristic not
always make sense depending on the system configuration and load).
The alloca usage was replaced with a fixed-size buffer.
For the fallback mechanism, the implementation uses heapsort. It is
simpler than quicksort, and it does not suffer from adversarial
inputs. With memory overcommit, it should be rarely triggered.
The drawback is mergesort requires O(n) extra space, and since it is
allocated with malloc the function is AS-signal-unsafe. It should be
feasible to change it to use mmap, although I am not sure how urgent
it is. The heapsort is also nonstable, so programs that require a
stable sort would still be subject to this latent issue.
The tst-qsort5 is removed since it will not create quicksort adversarial
inputs with the current qsort_r implementation.
Checked on x86_64-linux-gnu and aarch64-linux-gnu.
Reviewed-by: Florian Weimer <fweimer@redhat.com>
C23 adds a header <stdbit.h> with various functions and type-generic
macros for bit-manipulation of unsigned integers (plus macro defines
related to endianness). Implement this header for glibc.
The functions have both inline definitions in the header (referenced
by macros defined in the header) and copies with external linkage in
the library (which are implemented in terms of those macros to avoid
duplication). They are documented in the glibc manual. Tests, as
well as verifying results for various inputs (of both the macros and
the out-of-line functions), verify the types of those results (which
showed up a bug in an earlier version with the type-generic macro
stdc_has_single_bit wrongly returning a promoted type), that the
macros can be used at top level in a source file (so don't use ({})),
that they evaluate their arguments exactly once, and that the macros
for the type-specific functions have the expected implicit conversions
to the relevant argument type.
Jakub previously referred to -Wconversion warnings in type-generic
macros, so I've included a test with -Wconversion (but the only
warnings I saw and fixed from that test were actually in inline
functions in the <stdbit.h> header - not anything coming from use of
the type-generic macros themselves).
This implementation of the type-generic macros does not handle
unsigned __int128, or unsigned _BitInt types with a width other than
that of a standard integer type (and C23 doesn't require the header to
handle such types either). Support for those types, using the new
type-generic built-in functions Jakub's added for GCC 14, can
reasonably be added in a followup (along of course with associated
tests).
This implementation doesn't do anything special to handle C++, or have
any tests of functionality in C++ beyond the existing tests that all
headers can be compiled in C++ code; it's not clear exactly what form
this header should take in C++, but probably not one using macros.
DIS ballot comment AT-107 asks for the word "count" to be added to the
names of the stdc_leading_zeros, stdc_leading_ones,
stdc_trailing_zeros and stdc_trailing_ones functions and macros. I
don't think it's likely to be accepted (accepting any technical
comments would mean having an FDIS ballot), but if it is accepted at
the WG14 meeting (22-26 January in Strasbourg, starting with DIS
ballot comment handling) then there would still be time to update
glibc for the renaming before the 2.39 release.
The new functions and header are placed in the stdlib/ directory in
glibc, rather than creating a new toplevel stdbit/ or putting them in
string/ alongside ffs.
Tested for x86_64 and x86.
Verify that setjmp and longjmp work correctly between user contexts.
Arrange stacks for uctx_func1 and uctx_func2 so that ____longjmp_chk
works when setjmp and longjmp are called from different user contexts.
Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>
The previous check did not do anything because tmp_ptr already
points before run_ptr due to the way it is initialized.
Fixes commit e4d8117b82
("stdlib: Avoid another self-comparison in qsort").
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
The existing logic avoided internal stack overflow. To avoid
a denial-of-service condition with adversarial input, it is necessary
to fall over to heapsort if tail-recursing deeply, too, which does
not result in a deep stack of pending partitions.
The new test stdlib/tst-qsort5 is based on Douglas McIlroy's paper
on this subject.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
The previous implementation did not consistently apply the rule that
the child nodes of node K are at 2 * K + 1 and 2 * K + 2, or
that the parent node is at (K - 1) / 2.
Add an internal test that targets the heapsort implementation
directly.
Reported-by: Stepan Golosunov <stepan@golosunov.pp.ru>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
This patch adds a qsort and qsort_r to trigger the worst case
scenario for the quicksort (which glibc current lacks coverage).
The test is done with random input, dfferent internal types (uint8_t,
uint16_t, uint32_t, uint64_t, large size), and with
different set of element numbers.
Checked on x86_64-linux-gnu and i686-linux-gnu.
Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>
This patch removes the mergesort optimization on qsort implementation
and uses the introsort instead. The mergesort implementation has some
issues:
- It is as-safe only for certain types sizes (if total size is less
than 1 KB with large element sizes also forcing memory allocation)
which contradicts the function documentation. Although not required
by the C standard, it is preferable and doable to have an O(1) space
implementation.
- The malloc for certain element size and element number adds
arbitrary latency (might even be worse if malloc is interposed).
- To avoid trigger swap from memory allocation the implementation
relies on system information that might be virtualized (for instance
VMs with overcommit memory) which might lead to potentially use of
swap even if system advertise more memory than actually has. The
check also have the downside of issuing syscalls where none is
expected (although only once per execution).
- The mergesort is suboptimal on an already sorted array (BZ#21719).
The introsort implementation is already optimized to use constant extra
space (due to the limit of total number of elements from maximum VM
size) and thus can be used to avoid the malloc usage issues.
Resulting performance is slower due the usage of qsort, specially in the
worst-case scenario (partialy or sorted arrays) and due the fact
mergesort uses a slight improved swap operations.
This change also renders the BZ#21719 fix unrequired (since it is meant
to fix the sorted input performance degradation for mergesort). The
manual is also updated to indicate the function is now async-cancel
safe.
Checked on x86_64-linux-gnu.
Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>
Since the _FORTIFY_SOURCE feature uses some routines of Glibc, they need to
be excluded from the fortification.
On top of that:
- some tests explicitly verify that some level of fortification works
appropriately, we therefore shouldn't modify the level set for them.
- some objects need to be build with optimization disabled, which
prevents _FORTIFY_SOURCE to be used for them.
Assembler files that implement architecture specific versions of the
fortified routines were not excluded from _FORTIFY_SOURCE as there is no
C header included that would impact their behavior.
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Test minimum and maximum long long values, zero, 32bit crossover points, and
part of the range of long long values. Use '-fno-builtin' to ensure we are
testing the implementation.
Reviewed-by: Wilco Dijkstra <Wilco.Dijkstra@arm.com>
Test minimum and maximum long values, zero, and part of the range
of long values. Use '-fno-builtin' to ensure we are testing the
implementation.
Reviewed-by: Wilco Dijkstra <Wilco.Dijkstra@arm.com>
Test minimum and maximum int values, zero, and part of the range
of int values. Use '-fno-builtin' to ensure we are testing the
implementation.
Reviewed-by: Wilco Dijkstra <Wilco.Dijkstra@arm.com>
Reflow Makefile.
Sort using scripts/sort-makefile-lines.py.
No code generation changes observed in binary artifacts.
No regressions on x86_64 and i686.
We made tst-system.c depend on pthread, but that requires linking with
$(shared-thread-library). It does not fail under Linux because the
variable expands to nothing under Linux, but it fails for Hurd.
I tested verified via cross-compiling that "make check" now works
for Hurd.
Signed-off-by: Adam Yi <ayi@janestreet.com>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
C2x adds binary integer constants starting with 0b or 0B, and supports
those constants in strtol-family functions when the base passed is 0
or 2. Implement that strtol support for glibc.
As discussed at
<https://sourceware.org/pipermail/libc-alpha/2020-December/120414.html>,
this is incompatible with previous C standard versions, in that such
an input string starting with 0b or 0B was previously required to be
parsed as 0 (with the rest of the string unprocessed). Thus, as
proposed there, this patch adds 20 new __isoc23_* functions with
appropriate header redirection support. This patch does *not* do
anything about scanf %i (which will need 12 new functions per long
double variant, so 12, 24 or 36 depending on the glibc configuration),
instead leaving that for a future patch. The function names would
remain as __isoc23_* even if C2x ends up published in 2024 rather than
2023.
Making this change leads to the question of what should happen to
internal uses of these functions in glibc and its tests. The header
redirection (which applies for _GNU_SOURCE or any other feature test
macros enabling C2x features) has the effect of redirecting internal
uses but without those uses then ending up at a hidden alias (see the
comment in include/stdio.h about interaction with libc_hidden_proto).
It seems desirable for the default for internal uses to be the same
versions used by normal code using _GNU_SOURCE, so rather than doing
anything to disable that redirection, similar macro definitions to
those in include/stdio.h are added to the include/ headers for the new
functions.
Given that the default for uses in glibc is for the redirections to
apply, the next question is whether the C2x semantics are correct for
all those uses. Uses with the base fixed to 10, 16 or any other value
other than 0 or 2 can be ignored. I think this leaves the following
internal uses to consider (an important consideration for review of
this patch will be both whether this list is complete and whether my
conclusions on all entries in it are correct):
benchtests/bench-malloc-simple.c
benchtests/bench-string.h
elf/sotruss-lib.c
math/libm-test-support.c
nptl/perf.c
nscd/nscd_conf.c
nss/nss_files/files-parse.c
posix/tst-fnmatch.c
posix/wordexp.c
resolv/inet_addr.c
rt/tst-mqueue7.c
soft-fp/testit.c
stdlib/fmtmsg.c
support/support_test_main.c
support/test-container.c
sysdeps/pthread/tst-mutex10.c
I think all of these places are OK with the new semantics, except for
resolv/inet_addr.c, where the POSIX semantics of inet_addr do not
allow for binary constants; thus, I changed that file (to use
__strtoul_internal, whose semantics are unchanged) and added a test
for this case. In the case of posix/wordexp.c I think accepting
binary constants is OK since POSIX explicitly allows additional forms
of shell arithmetic expressions, and in stdlib/fmtmsg.c SEV_LEVEL is
not in POSIX so again I think accepting binary constants is OK.
Functions such as __strtol_internal, which are only exported for
compatibility with old binaries from when those were used in inline
functions in headers, have unchanged semantics; the __*_l_internal
versions (purely internal to libc and not exported) have a new
argument to specify whether to accept binary constants.
As well as for the standard functions, the header redirection also
applies to the *_l versions (GNU extensions), and to legacy functions
such as strtoq, to avoid confusing inconsistency (the *q functions
redirect to __isoc23_*ll rather than needing their own __isoc23_*
entry points). For the functions that are only declared with
_GNU_SOURCE, this means the old versions are no longer available for
normal user programs at all. An internal __GLIBC_USE_C2X_STRTOL macro
is used to control the redirections in the headers, and cases in glibc
that wish to avoid the redirections - the function implementations
themselves and the tests of the old versions of the GNU functions -
then undefine and redefine that macro to allow the old versions to be
accessed. (There would of course be greater complexity should we wish
to make any of the old versions into compat symbols / avoid them being
defined at all for new glibc ABIs.)
strtol_l.c has some similarity to strtol.c in gnulib, but has already
diverged some way (and isn't listed at all at
https://sourceware.org/glibc/wiki/SharedSourceFiles unlike strtoll.c
and strtoul.c); I haven't made any attempts at gnulib compatibility in
the changes to that file.
I note incidentally that inttypes.h and wchar.h are missing the
__nonnull present on declarations of this family of functions in
stdlib.h; I didn't make any changes in that regard for the new
declarations added.
If using -D_FORITFY_SOURCE=3 (in my case, I've patched GCC to add
=3 instead of =2 (we've done =2 for years in Gentoo)), building
glibc tests will fail on testmb like:
```
<command-line>: error: "_FORTIFY_SOURCE" redefined [-Werror]
<built-in>: note: this is the location of the previous definition
cc1: all warnings being treated as errors
make[2]: *** [../o-iterator.mk:9: /var/tmp/portage/sys-libs/glibc-2.36/work/build-x86-x86_64-pc-linux-gnu-nptl/stdlib/testmb.o] Error 1
make[2]: *** Waiting for unfinished jobs....
```
It's just because we're always setting -D_FORTIFY_SOURCE=2
rather than unsetting it first. If F_S is already 2, it's harmless,
but if it's another value (say, 1, or 3), the compiler will bawk.
(I'm not aware of a reason this couldn't be tested with =3,
but the toolchain support is limited for that (too new), and we want
to run the tests everywhere possible.)
Signed-off-by: Sam James <sam@gentoo.org>
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
clang emits an warning when a double alias redirection is used, to warn
the the original symbol will be used even when weak definition is
overridden. However, this is a common pattern for weak_alias, where
multiple alias are set to same symbol.
Reviewed-by: Fangrui Song <maskray@google.com>
Rather than buffering 16 MiB of entropy in userspace (by way of
chacha20), simply call getrandom() every time.
This approach is doubtlessly slower, for now, but trying to prematurely
optimize arc4random appears to be leading toward all sorts of nasty
properties and gotchas. Instead, this patch takes a much more
conservative approach. The interface is added as a basic loop wrapper
around getrandom(), and then later, the kernel and libc together can
work together on optimizing that.
This prevents numerous issues in which userspace is unaware of when it
really must throw away its buffer, since we avoid buffering all
together. Future improvements may include userspace learning more from
the kernel about when to do that, which might make these sorts of
chacha20-based optimizations more possible. The current heuristic of 16
MiB is meaningless garbage that doesn't correspond to anything the
kernel might know about. So for now, let's just do something
conservative that we know is correct and won't lead to cryptographic
issues for users of this function.
This patch might be considered along the lines of, "optimization is the
root of all evil," in that the much more complex implementation it
replaces moves too fast without considering security implications,
whereas the incremental approach done here is a much safer way of going
about things. Once this lands, we can take our time in optimizing this
properly using new interplay between the kernel and userspace.
getrandom(0) is used, since that's the one that ensures the bytes
returned are cryptographically secure. But on systems without it, we
fallback to using /dev/urandom. This is unfortunate because it means
opening a file descriptor, but there's not much of a choice. Secondly,
as part of the fallback, in order to get more or less the same
properties of getrandom(0), we poll on /dev/random, and if the poll
succeeds at least once, then we assume the RNG is initialized. This is a
rough approximation, as the ancient "non-blocking pool" initialized
after the "blocking pool", not before, and it may not port back to all
ancient kernels, though it does to all kernels supported by glibc
(≥3.2), so generally it's the best approximation we can do.
The motivation for including arc4random, in the first place, is to have
source-level compatibility with existing code. That means this patch
doesn't attempt to litigate the interface itself. It does, however,
choose a conservative approach for implementing it.
Cc: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Cristian Rodríguez <crrodriguez@opensuse.org>
Cc: Paul Eggert <eggert@cs.ucla.edu>
Cc: Mark Harris <mark.hsj@gmail.com>
Cc: Eric Biggers <ebiggers@kernel.org>
Cc: linux-crypto@vger.kernel.org
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
The basic tst-arc4random-chacha20.c checks if the output of ChaCha20
implementation matches the reference test vectors from RFC8439.
The tst-arc4random-fork.c check if subprocesses generate distinct
streams of randomness (if fork handling is done correctly).
The tst-arc4random-stats.c is a statistical test to the randomness of
arc4random, arc4random_buf, and arc4random_uniform.
The tst-arc4random-thread.c check if threads generate distinct streams
of randomness (if function are thread-safe).
Checked on x86_64-linux-gnu, aarch64-linux, and powerpc64le-linux-gnu.
Co-authored-by: Florian Weimer <fweimer@redhat.com>
Checked on x86_64-linux-gnu and aarch64-linux-gnu.
The implementation is based on scalar Chacha20 with per-thread cache.
It uses getrandom or /dev/urandom as fallback to get the initial entropy,
and reseeds the internal state on every 16MB of consumed buffer.
To improve performance and lower memory consumption the per-thread cache
is allocated lazily on first arc4random functions call, and if the
memory allocation fails getentropy or /dev/urandom is used as fallback.
The cache is also cleared on thread exit iff it was initialized (so if
arc4random is not called it is not touched).
Although it is lock-free, arc4random is still not async-signal-safe
(the per thread state is not updated atomically).
The ChaCha20 implementation is based on RFC8439 [1], omitting the final
XOR of the keystream with the plaintext because the plaintext is a
stream of zeros. This strategy is similar to what OpenBSD arc4random
does.
The arc4random_uniform is based on previous work by Florian Weimer,
where the algorithm is based on Jérémie Lumbroso paper Optimal Discrete
Uniform Generation from Coin Flips, and Applications (2013) [2], who
credits Donald E. Knuth and Andrew C. Yao, The complexity of nonuniform
random number generation (1976), for solving the general case.
The main advantage of this method is the that the unit of randomness is not
the uniform random variable (uint32_t), but a random bit. It optimizes the
internal buffer sampling by initially consuming a 32-bit random variable
and then sampling byte per byte. Depending of the upper bound requested,
it might lead to better CPU utilization.
Checked on x86_64-linux-gnu, aarch64-linux, and powerpc64le-linux-gnu.
Co-authored-by: Florian Weimer <fweimer@redhat.com>
Reviewed-by: Yann Droneaud <ydroneaud@opteya.com>
[1] https://datatracker.ietf.org/doc/html/rfc8439
[2] https://arxiv.org/pdf/1304.1916.pdf
mbstows is defined if dst is NULL and is defined to special cased if
dst is NULL so the fortify objsize check if incorrect in that case.
Tested on x86-64 linux.
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
