forked from Imagelibrary/binutils-gdb
fcfd8a4f23
It is possible, when creating a shared memory segment (i.e. with shmget), that the id of the segment will be zero. When looking at the segment in /proc/PID/smaps, the inode field of the entry holds the shared memory segment id. And so, it can be the case that an entry (in the smaps file) will have an inode of zero. When GDB generates a core file, with the generate-core-file (or its gcore alias) command, the shared memory segment should be written into the core file. Fedora GDB has, since 2008, carried a patch that tests this case. There is no fix for GDB associated with the test, and unfortunately, the motivation for the test has been lost to the mists of time. This likely means that a fix was merged upstream without a suitable test, but I've not been able to find and relevant commit. The test seems to be checking that the shared memory segment with id zero, is being written to the core file. While looking at this test and trying to work out if it should be posted upstream, I saw that GDB does appear to write the shared memory segment into the core file (as expected), which is good. However, GDB still isn't getting this case exactly right, there appears to be no NT_FILE entry for the shared memory mapping if the mapping had an id of zero. In gcore_memory_sections (gcore.c) we call back into linux-tdep.c (via the gdbarch_find_memory_regions call) to correctly write the shared memory segment into the core file, however, in linux_make_mappings_corefile_notes, when we use linux_find_memory_regions_full to create the NT_FILE note, we call back in to dump_note_entry_p for each mapping, and in here we reject any mapping with a zero inode. The result of this, is that, for a shared memory segment with a non-zero id, after loading the core file, the shared memory segment will appear in the 'proc info mappings' output. But, for a shared memory segment with a zero id, the segment will not appear in the 'proc info mappings' output. I initially tried just dropping the inode check in this function (see previous commit1e21c846c2, which I then reverted in commit998165ba99. The problem with dropping the inode check is that the special kernel mappings, e.g. '[vvar]' would now get a NT_FILE entry. In fact, any special entry except '[vdso]' and '[vsyscall]' which are specifically checked for in dump_note_entry_p would get a NT_FILE entry, which is not correct. So, instead, I propose that if the inode is zero, and the filename starts with '[' and finished with ']' then we should not create a NT_FILE entry. But otherwise a zero inode should not prevent a NT_FILE entry being created. The test for this change is a bit tricky. The original Fedora test (mentioned above) has a loop that tries to grab the shared memory mapping with id zero. This was, unfortunately, not very reliable. I tried to make this more reliable by going multi-threaded, and waiting for longer, see my proposal here: https://inbox.sourceware.org/gdb-patches/0d389b435cbb0924335adbc9eba6cf30b4a2c4ee.1741776651.git.aburgess@redhat.com But this was still not great. On further testing this was only passing (i.e. managing to find the shared memory mapping with id zero) about 60% of the time. However, I realised that GDB finds the shared memory id by reading the /proc/PID/smaps file. But we don't really _need_ the shared memory id for anything, we just use the value (as an inode) to decide if the segment should be included in the core file or not. The id isn't even written to the core file. So, if we could intercept the read of the smaps file, then maybe, we could lie to GDB, and tell it that the id was zero, and then see how GDB handles this. And luckily, we can do that using a preload library! We already have a test that uses a preload library to modify GDB, see gdb.threads/attach-slow-waitpid.exp. So, I have created a new preload library. This one intercepts open, open64, close, read, and pread. When GDB attempts to open /proc/PID/smaps, the library spots this and loads the file contents into a memory buffer. The buffer is then modified to change the id of any shared memory mapping to zero. Any reads from this file are served from the modified memory buffer. I tested on x86-64, AArch64, PPC, s390, and ARM, all running various versions of GNU/Linux. The requirement for open64() came from my ARM testing. The other targets used plain open(). And so, the test is now simple. Start GDB with the preload library in place, start the inferior and generate a core file. Then restart GDB, load the core file, and check the shared memory mapping was included. This test will fail with an unpatched GDB, and succeed with the patch applied. Tested-By: Guinevere Larsen <guinevere@redhat.com>
64 lines
1.5 KiB
C
64 lines
1.5 KiB
C
/* This testcase is part of GDB, the GNU debugger.
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Copyright 2025 Free Software Foundation, Inc.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>. */
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#include <sys/ipc.h>
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#include <sys/shm.h>
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#include <stdio.h>
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#include <errno.h>
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#include <stdlib.h>
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#include <unistd.h>
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#include <assert.h>
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#include <time.h>
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void
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breakpt (void)
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{
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/* Nothing. */
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}
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int
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main (void)
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{
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/* Create a shared memory mapping. */
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int sid = shmget (IPC_PRIVATE, 0x1000, IPC_CREAT | IPC_EXCL | 0777);
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if (sid == -1)
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{
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perror ("shmget");
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exit (1);
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}
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/* Attach the shared memory mapping. */
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void *addr = shmat (sid, NULL, SHM_RND);
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if (addr == (void *) -1L)
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{
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perror ("shmat");
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exit (1);
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}
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breakpt ();
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/* Mark the shared memory mapping as deleted -- once the last user
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has finished with it. */
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if (shmctl (sid, IPC_RMID, NULL) != 0)
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{
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perror ("shmctl");
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exit (1);
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}
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return 0;
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}
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