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elf: Restore support for _r_debug interpositions and copy relocations

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The changes in commit a93d9e03a3
("Extend struct r_debug to support multiple namespaces [BZ #15971]")
break the dyninst dynamic instrumentation tool.  It brings its
own definition of _r_debug (rather than a declaration).

Furthermore, it turns out it is rather hard to use the proposed
handshake for accessing _r_debug via DT_DEBUG. If applications want
to access _r_debug, they can do so directly if the relevant code has
been built as PIC.  To protect against harm from accidental copy
relocations due to linker relaxations, this commit restores copy
relocation support by adjusting both copies if interposition or
copy relocations are in play.  Therefore, it is possible to
use a hidden reference in ld.so to access _r_debug.

Only perform the copy relocation initialization if libc has been
loaded.  Otherwise, the ld.so search scope can be empty, and the
lookup of the _r_debug symbol mail fail.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
This commit is contained in:
Florian Weimer
2025-07-04 21:46:30 +02:00
parent 8329939a37
commit ea85e7d550
9 changed files with 94 additions and 3 deletions
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2
include/link.h
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@@ -365,6 +365,8 @@ struct auditstate
dynamic linker. */
extern struct r_debug_extended _r_debug_extended attribute_hidden;
rtld_hidden_proto (_r_debug)
#if __ELF_NATIVE_CLASS == 32
# define symbind symbind32
# define LA_SYMBIND "la_symbind32"