Hi Stephen,
On Fri, Apr 4, 2025 at 11:43 AM Stephen Brennan
<stephen.s.bren...@oracle.com> wrote:
>
> Stephen Brennan <stephen.s.bren...@oracle.com> writes:
> > Tao Liu <l...@redhat.com> writes:
> >> Hi Stephen,
> >>
> >> Thanks for reporting the issue and patch. Please check if I understood
> >> you correctly. The correct output you are expecting in your case is:
> >>
> >> crash> sym ata_dummy_port_ops
> >> ffffffffc0a71580 (?) ata_dummy_port_ops [libata]
> >> crash> mod -s libata
> >> MODULE NAME TEXT_BASE SIZE
> >> OBJECT FILE
> >> ffffffffc0a7b640 libata ffffffffc0a47000 520192
> >>
> >> /usr/lib/debug/lib/modules/6.12.0-0.11.8.el9uek.x86_64/kernel/drivers/ata/libata.ko.debug
> >> crash> sym ata_dummy_port_ops
> >> ffffffffc0a71580 (B) ata_dummy_port_ops [libata]
> >> crash> p/x &ata_dummy_port_ops
> >> $1 = 0xffffffffc0a6fe80 <<-------- should be 0xffffffffc0a71580,
> >> same as sym's output, right?
> >
> > Yes, that's correct.
> >
> >> If that is the case, then after applied your patch, the issue still
> >> exists on my machine:
> >>
> >> crash> sym fuse_miscdevice
> >> ffffffffc05f7fe0 (?) fuse_miscdevice [fuse]
> >> crash> mod -s fuse
> >> MODULE NAME TEXT_BASE
> >> SIZE OBJECT FILE
> >> ffffffffc05f8dc0 fuse ffffffffc05da000
> >> 233472
> >> /usr/lib/debug/lib/modules/6.11.5-300.fc41.x86_64/kernel/fs/fuse/fuse.ko.debug
> >> crash> sym fuse_miscdevice
> >> ffffffffc05f7fe0 (b) fuse_miscdevice [fuse]
> >> crash> p/x &fuse_miscdevice
> >> $1 = 0xffffffffc05daf20 << ------ unmatch with the previous value.
> >
> > Indeed, I checked with fuse on Fedora (6.11.4-301.fc41.x86_64, so almost
> > the same version). And I see the same behavior with fuse_miscdevice.
> >
> > In your case and mine, the offset is 0x1d0c0, which could be a clue to
> > how this mismatch happens.
> >
> >> The .data section of fuse.ko.debug have non-zero address:
> >>
> >> $ readelf -S
> >> /usr/lib/debug/lib/modules/6.11.5-300.fc41.x86_64/kernel/fs/fuse/fuse.ko.debug
> >> Section Headers:
> >> [Nr] Name Type Address Offset
> >> Size EntSize Flags Link Info Align
> >> ...
> >> [50] .data NOBITS 0000000000000e20 00000100
> >> 000000000000080f 0000000000000000 WA 0 0 32
> >>
> >> Could you please re-check your patch for this, or is there something
> >> I'm missing?
> >
> > From what I can tell, the important difference here may be that the
> > symbol you selected is a static variable (which becomes a local ELF
> > symbol). Apparently, my patch only resolves the issue for *global
> > variables*, but not for local variables.
> >
> > $ eu-readelf -s /usr/lib/debug/lib/modules/$(uname
> > -r)/kernel/fs/fuse/fuse.ko.debug | grep -P "fuse_(miscdevice|mutex)"
> > 85: 0000000000000100 80 OBJECT LOCAL DEFAULT 50
> > fuse_miscdevice
> > 1041: 00000000000005c0 32 OBJECT GLOBAL DEFAULT 50 fuse_mutex
> >
> > Here is the behavior of crash's master branch on these two symbols:
> >
> > crash> sym fuse_miscdevice
> > ffffffffc02fafe0 (?) fuse_miscdevice [fuse]
> > crash> sym fuse_mutex
> > ffffffffc02fb4a0 (?) fuse_mutex [fuse]
> > crash> mod -s fuse
> > MODULE NAME TEXT_BASE
> > SIZE OBJECT FILE
> > ffffffffc02fbdc0 fuse ffffffffc02dd000
> > 233472
> > /usr/lib/debug/lib/modules/6.11.4-301.fc41.x86_64/kernel/fs/fuse/fuse.ko.debug
> > crash> sym fuse_miscdevice
> > ffffffffc02fafe0 (b) fuse_miscdevice [fuse]
> > crash> sym fuse_mutex
> > ffffffffc02fb4a0 (B) fuse_mutex [fuse]
> > crash> p/x &fuse_miscdevice
> > $1 = 0xffffffffc02ddf20
> > crash> p/x &fuse_mutex
> > $2 = 0xffffffffc02fa680
> >
> > Both fuse_miscdevice, and fuse_mutex are mismatched (with different
> > offsets). And now, here's the behavior of crash with my patch:
> >
> > crash> sym fuse_miscdevice
> > ffffffffc02fafe0 (?) fuse_miscdevice [fuse]
> > crash> sym fuse_mutex
> > ffffffffc02fb4a0 (?) fuse_mutex [fuse]
> > crash> mod -s fuse
> > MODULE NAME TEXT_BASE
> > SIZE OBJECT FILE
> > ffffffffc02fbdc0 fuse ffffffffc02dd000
> > 233472
> > /usr/lib/debug/lib/modules/6.11.4-301.fc41.x86_64/kernel/fs/fuse/fuse.ko.debug
> > crash> sym fuse_miscdevice
> > ffffffffc02fafe0 (b) fuse_miscdevice [fuse]
> > crash> sym fuse_mutex
> > ffffffffc02fb4a0 (B) fuse_mutex [fuse]
> > crash> p/x &fuse_miscdevice
> > $1 = 0xffffffffc02ddf20
> > crash> p/x &fuse_mutex
> > $2 = 0xffffffffc02fb4a0
> >
> > Here the global fuse_mutex is correct, but the static/local
> > fuse_miscdevice is still incorrect. This indicates that there's more yet
> > to fix. (But at least my patch does part of the work!)
> >
> > I'm wondering whether this has anything to do with ELF relocations.
>
> Hello Tao,
>
> I've continued debugging and verified that the root cause for this was
> GDB performing relocations to the DWARF info incorrectly. At the bottom
> of this email I've attached my patch, which you can apply to the master
> branch to test out my proposed fix. The patch file also contains a
> longer explanation of what I believe the issue is.
>
> However, since it updates the gdb-16.2.patch file, it's quite difficult
> to understand. Thus, I'll also provide a diff here of the relevant GDB
> source file.
>
> I'm not certain how the GDB patches are normally updated & reviewed. I
> also recognize that this change is a bit messy, so I'd appreciate your
> feedback as well.
Thanks a lot for your efforts on this. I'm looking into your patch,
please wait a while.
Thanks,
Tao Liu
>
> Thank you,
> Stephen
>
> --- tmp/gdb-16.2/gdb/symfile.c 2025-04-03 15:38:26.093760270 -0700
> +++ gdb-16.2/gdb/symfile.c 2025-04-03 15:07:43.239691104 -0700
> @@ -341,8 +341,13 @@ place_section (bfd *abfd, asection *sect
> return;
>
> /* If the user specified an offset, honor it. */
> - if (offsets[gdb_bfd_section_index (abfd, sect)] != 0)
> + if (offsets[gdb_bfd_section_index (abfd, sect)] != 0) {
> + /* addr_info_make_relative() subtracts out the section VMA. But if the
> user
> + * specified an offset, they have already taken this into account. Add it
> + * back in */
> + offsets[gdb_bfd_section_index (abfd, sect)] += bfd_section_vma(sect);
> return;
> + }
>
> /* Otherwise, let's try to find a place for the section. */
> start_addr = (lowest + align - 1) & -align;
> @@ -628,35 +633,8 @@ default_symfile_offsets (struct objfile
> if ((bfd_get_file_flags (objfile->obfd.get ()) & (EXEC_P | DYNAMIC)) == 0)
> {
> bfd *abfd = objfile->obfd.get ();
> - asection *cur_sec;
> + asection *cur_sec = NULL;
>
> - for (cur_sec = abfd->sections; cur_sec != NULL; cur_sec =
> cur_sec->next)
> - /* We do not expect this to happen; just skip this step if the
> - relocatable file has a section with an assigned VMA. */
> - if (bfd_section_vma (cur_sec) != 0
> - /*
> - * Kernel modules may have some non-zero VMAs, i.e., like the
> - * __ksymtab and __ksymtab_gpl sections in this example:
> - *
> - * Section Headers:
> - * [Nr] Name Type Address
> Offset
> - * Size EntSize Flags Link Info
> Align
> - * ...
> - * [ 8] __ksymtab PROGBITS 0000000000000060
> 0000ad90
> - * 0000000000000010 0000000000000000 A 0 0
> 16
> - * [ 9] .rela__ksymtab RELA 0000000000000000
> 0000ada0
> - * 0000000000000030 0000000000000018 43 8
> 8
> - * [10] __ksymtab_gpl PROGBITS 0000000000000070
> 0000add0
> - * 00000000000001a0 0000000000000000 A 0 0
> 16
> - * ...
> - *
> - * but they should be treated as if they are NULL.
> - */
> - && strncmp (bfd_section_name (cur_sec), "__k", 3) != 0)
> - break;
> -
> - if (cur_sec == NULL)
> - {
> section_offsets &offsets = objfile->section_offsets;
>
> /* Pick non-overlapping offsets for sections the user did not
> @@ -704,7 +682,6 @@ default_symfile_offsets (struct objfile
> offsets[cur_sec->index]);
> offsets[cur_sec->index] = 0;
> }
> - }
> }
>
> /* Remember the bfd indexes for the .text, .data, .bss and
>
>
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