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authorJoel Brobecker <brobecker@adacore.com>2013-11-05 07:01:45 -0500
committerJoel Brobecker <brobecker@adacore.com>2013-11-13 06:43:57 +0400
commit846060dfd8ec2bf0e78b4049a89b51438bfe0072 (patch)
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parent7d4df6a4e13f9f15c26ac132775d7ab570b38456 (diff)
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crash while re-reading symbols from objfile on ppc-aix.
This patch aims at fixing the following problem, where the user: . debugs its program . makes a modification and rebuilds it *without exiting the debugger* . returns to its debugging session and restarts the inferior In that situation, the debugger notices that the underlying executable has changed and that re-reading its symbols is needed. Shortly after displaying a message informing the user of the situation, GDB crashes: (gdb) run [...] `/[...]/dest' has changed; re-reading symbols. zsh: 13434922 segmentation fault (core dumped) The crash occurs while trying to allocate some memory on the bfd_bfd obstack. But, at some point in time, the whole obstack data gets corrupted, nullified. So the memory allocation fails trying to call a function at a NULL address. (side note: when debugging GDB in GDB, top-gdb reports a SIGILL, while the shell makes it look like it was a SIGSEGV - the discrepancy is not critical to the investigation and therefore was not explored) The corruption occurred because the region where the per_bfd data got free'ed nearly after it got allocated! This is what happens, in chronological order (see reread_symbols): 1. GDB notices that the executable has changed, decides to re-read its symbols. 2. Opens a new bfd, unrefs the old one 3. Calls set_objfile_per_bfd (objfile); 4. Re-initializes the objfile's obstack: obstack_init (&objfile->objfile_obstack); I think that the normal behavior for set_objfile_per_bfd would be to search for already-allocated shared per_bfd data, and allocate new one if not found. The critical difference between a platform such as x86_64-linuxe where it works, and ppc-aix, where it doesn't lies in the fact that bfd-data sharing is not activated on ppc-aix, and as a result, the per-bfd data gets allocated on the objfile's obstack instead of in the bfd objalloc: /* If the object requires gdb to do relocations, we simply fall back to not sharing data across users. These cases are rare enough that this seems reasonable. */ if (abfd != NULL && !gdb_bfd_requires_relocations (abfd)) { storage = bfd_zalloc (abfd, sizeof (struct objfile_per_bfd_storage)); set_bfd_data (abfd, objfiles_bfd_data, storage); } else storage = OBSTACK_ZALLOC (&objfile->objfile_obstack, struct objfile_per_bfd_storage); Allocating that per_bfd storage is of course nearly useless since we end up free-ing right after in step (4) above. Eventually, the memory region ends up being re-used, hence the corruption leading to the crash. This fix was simply to move the call to set_objfile_per_bfd after the objfile's obstack re-initialization. gdb/ChangeLog: * symfile.c (reread_symbols): Move call to set_objfile_per_bfd after re-initialization of OBJFILE's obstack.
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