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Re: [PATCH] Catch exception on solib_svr4_r_ldsomap
- From: Pedro Alves <palves at redhat dot com>
- To: Sergio Durigan Junior <sergiodj at redhat dot com>
- Cc: GDB Patches <gdb-patches at sourceware dot org>
- Date: Tue, 31 Mar 2015 12:30:14 +0100
- Subject: Re: [PATCH] Catch exception on solib_svr4_r_ldsomap
- Authentication-results: sourceware.org; auth=none
- References: <871tke2b6a dot fsf at redhat dot com> <55152B9F dot 4090606 at redhat dot com> <87k2y2qdbu dot fsf_-_ at redhat dot com>
On 03/27/2015 10:34 PM, Sergio Durigan Junior wrote:
> On Friday, March 27 2015, Pedro Alves wrote:
>> On 03/25/2015 12:06 AM, Sergio Durigan Junior wrote:> Hi,
>>> While hacking the coredump_filter patch, I noticed that, when you load a
>>> corefile on GDB and receive a "Cannot access memory at address..."
>>> message, gdb_core_cmd will fail and return -1, which means that some
>>> fatal error happened.
>>> Unfortunately, this kind of message does not mean that the user cannot
>>> continue debugging with the corefile; it meant that some memory region
>>> (sometimes not important) was inaccessible. Given that
>>> gcore_create_callback, nowadays, will dump memory regions if they don't
>>> have the 'read' permission set (but have any other permission set), this
>>> kind of error can be expected sometimes.
>> So, gdb itself errors and stops processing the core?
> No, GDB does not "error and stop", but some testcases do that.
Well, it clearly does do that. Hence your new patch. :-)
>> I think I don't understand. :-) Can you please show an
>> example session? Did GDB continue processing the core when
>> it printed that error, or was it just a warning and it continued?
I meant "Did GDB stop processing the core", of course.
> Sure, sorry for not sending the example session before! Here is the
> pertinent part:
> (gdb) core /home/sergio/work/src/git/binutils-gdb/rhbz1085906-coredump-filter/build-64-3/gdb/testsuite/gdb.base/non-private-anon.gcore
> [New LWP 28468]
> Cannot access memory at address 0x355fc21148
> Cannot access memory at address 0x355fc21140
> (gdb) FAIL: gdb.base/coredump-filter.exp: loading and testing corefile for non-Private-Anonymous: load core
> FAIL: gdb.base/coredump-filter.exp: loading and testing corefile for non-Private-Anonymous: loading /home/sergio/work/src/git/binutils-gdb/rhbz1085906-coredump-filter/build-64-3/gdb/testsuite/gdb.base/non-private-anon.gcore
> spawn /home/sergio/work/src/git/binutils-gdb/rhbz1085906-coredump-filter/build-64-3/gdb/testsuite/../../gdb/gdb -nw -nx -data-directory /home/sergio/work/src/git/binutils-gdb/rhbz1085906-coredump-filter/build-64-3/gdb/testsuite/../data-directory
> GDB correctly loaded the corefile (despite the warnings), and the
The error made the rest of core_open be skipped: if the core is still
loaded, gdb is potentially in an inconsistent state at this point.
I'd think we should completely discard the core/target if something
errors out. And then if we can be tolerant to specific parts of
loading a core failing, we should handle those before the error escapes
out of core_open. We do something like that
already (note core_close_cleanup and the TRY/CATCH'S), but it's clearly
not complete. After:
... several things can throw and let an exception escape.
> Right, so I took some time and found the right fix, I think. As we
> agreed above, the fact that GDB is not printing the "Core was generated
> by..." message is really strange, so I decided to investigate why it is
> doing that.
> The answer is that we are forgetting to check for an exception on
> solib_svr4_r_ldsomap. When loading the corefile, GDB calls this
> function, which then calls read_memory_unsigned_integer, which throws an
> error. This error is not being caught by the function, so it propagates
> until the main loop catches it. The fix is obvious: we should catch
> this regression and continue in the function. With it, GDB now
> correctly prints the "Core was generated by..." message, and the patch
> to adjust gdb_core_cmd is no longer needed.
> Regression-tested on Fedora 20 for x86_64, i686 and native-gdbserver.
> Does that make more sense now?
Yes, this is OK.