Make sure the size parameter to read and write VFS methods in the unix VFS

do not become too big or go negative.  This was not actually possible in the
current code.  The checks are added to make sure some future bug does not
make it possible.

FossilOrigin-Name: daebe3bd2d9bd7b6f876a8110cf5045eb3fee078

manifest

@@ -1,5 +1,5 @@
-C Ensure\sthat\sthe\svalue\sreturned\sby\sxSectorSize()\sis\sreasonable\s(currently\sdefined\sas\sbetween\s2^5\sand\s2^16\sbytes)\sbefore\susing\sit\sto\scalculate\sthe\samount\sof\spadding\sto\sadd\sto\sa\swal\sfile.
-D 2012-10-01T06:50:55.576
+C Make\ssure\sthe\ssize\sparameter\sto\sread\sand\swrite\sVFS\smethods\sin\sthe\sunix\sVFS\ndo\snot\sbecome\stoo\sbig\sor\sgo\snegative.\s\sThis\swas\snot\sactually\spossible\sin\sthe\ncurrent\scode.\s\sThe\schecks\sare\sadded\sto\smake\ssure\ssome\sfuture\sbug\sdoes\snot\nmake\sit\spossible.
+D 2012-10-01T12:16:26.771
 F Makefile.arm-wince-mingw32ce-gcc d6df77f1f48d690bd73162294bbba7f59507c72f
 F Makefile.in 5f4f26109f9d80829122e0e09f9cda008fa065fb
 F Makefile.linux-gcc 91d710bdc4998cb015f39edf3cb314ec4f4d7e23
@@ -160,7 +160,7 @@ F src/notify.c 976dd0f6171d4588e89e874fcc765e92914b6d30
 F src/os.c e1acdc09ff3ac2412945cca9766e2dcf4675f31c
 F src/os.h 027491c77d2404c0a678bb3fb06286f331eb9b57
 F src/os_common.h 92815ed65f805560b66166e3583470ff94478f04
-F src/os_unix.c 69b2fe66316524eebf5f1ce85c1fdfe2952307e9
+F src/os_unix.c a5a45a2857c43b37bac145b521064a85a544cd7a
 F src/os_win.c 90c7a1fe2698867555ba4266f5bd436c85d0d1dc
 F src/pager.c 9f5f2823594cc2848e151510f726af02896485b5
 F src/pager.h bdbc379557eb2e233dfec10986b3086877e72db7
@@ -1018,7 +1018,7 @@ F tool/vdbe-compress.tcl f12c884766bd14277f4fcedcae07078011717381
 F tool/warnings-clang.sh f6aa929dc20ef1f856af04a730772f59283631d4
 F tool/warnings.sh fbc018d67fd7395f440c28f33ef0f94420226381
 F tool/win/sqlite.vsix 67d8a99aceb56384a81b3f30d6c71743146d2cc9
-P 62225b4a4c4bfe1820ef54cb202edf2cd866429f
-R 09d3e5b05c9dfd1b20715f593e6818b6
-U dan
-Z 07828d460d32254f1342208677faf52f
+P 6b4ff83bff07d427af585c9fd03be90abf2fc82f
+R 43398c1dd9f4f691234256568baab7a2
+U drh
+Z 321bb2af8cac9de7aef8d0d7efaf3889

manifest.uuid

@@ -1 +1 @@
-6b4ff83bff07d427af585c9fd03be90abf2fc82f
+daebe3bd2d9bd7b6f876a8110cf5045eb3fee078

src/os_unix.c

@@ -3010,6 +3010,8 @@ static int seekAndRead(unixFile *id, sqlite3_int64 offset, void *pBuf, int cnt){
   i64 newOffset;
 #endif
   TIMER_START;
+  assert( cnt==(cnt&0x1ffff) );
+  cnt &= 0x1ffff;
   do{
 #if defined(USE_PREAD)
     got = osPread(id->h, pBuf, cnt, offset);
@@ -3099,6 +3101,8 @@ static int seekAndWrite(unixFile *id, i64 offset, const void *pBuf, int cnt){
 #if (!defined(USE_PREAD) && !defined(USE_PREAD64))
   i64 newOffset;
 #endif
+  assert( cnt==(cnt&0x1ffff) );
+  cnt &= 0x1ffff;
   TIMER_START;
 #if defined(USE_PREAD)
   do{ got = osPwrite(id->h, pBuf, cnt, offset); }while( got<0 && errno==EINTR );