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([2804:14d:8084:993e:22e8:4ed1:527a:55c0]) by smtp.gmail.com with ESMTPSA id af79cd13be357-939d3b2d010sm39536485a.23.2026.09.09.04.19.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 09 Sep 2026 04:19:57 -0700 (PDT) Message-ID: <1e0c3148-dce7-4c8e-ab94-1eac2019dfc0@redhat.com> Date: Wed, 9 Sep 2026 08:19:55 -0300 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] [gdb/tui] Don't disassemble non-code sections and section holes To: Tom de Vries , gdb-patches@sourceware.org References: <20260819143514.2221499-1-tdevries@suse.de> <7c55b686-2067-4dbd-8eaa-c7a77ad11afa@redhat.com> <3af17c22-ef82-4825-8c49-702466da7d43@suse.de> From: Guinevere Larsen In-Reply-To: <3af17c22-ef82-4825-8c49-702466da7d43@suse.de> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: O4Q2w_XbxXWMKcwT0BIg_-HLKqWRtIaR7X-r4SSnrD0_1788952799 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-BeenThere: gdb-patches@sourceware.org X-Mailman-Version: 2.1.30 Precedence: list List-Id: Gdb-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: gdb-patches-bounces~public-inbox=simark.ca@sourceware.org On 9/9/26 7:41 AM, Tom de Vries wrote: > On 9/4/26 1:39 PM, Guinevere Larsen wrote: >> On 8/19/26 11:35 AM, Tom de Vries wrote: >>> I was investigating TUI behaviour on ppc64-linux using the >>> executable from >>> test-case gdb.tui/basic.exp, and after scrolling in the asm window >>> ended up >>> with: >>> ... >>> +----------------------------------------------------------------------------+ >>> >>> |   0x920 <__libc_start_main@plt+4> b       0x8f0 >>> <__glink_PLTresolve>       | >>> |   0x924 <__gmon_start__@plt>      li >>> r0,1                             | >>> |   0x928 <__gmon_start__@plt+4>    b       0x8f0 >>> <__glink_PLTresolve>       | >>> |   0x92c <__cxa_finalize@plt>      li >>> r0,2                             | >>> |   0x930 <__cxa_finalize@plt+4>    b       0x8f0 >>> <__glink_PLTresolve>       | >>> |   0x934 <._fini>                  mflr >>> r0                               | >>> |   0x938 <._fini+4>                std >>> r0,16(r1)                        | >>> |   0x93c <._fini+8>                stdu >>> r1,-112(r1)                      | >>> |   0x940 <._fini+12>               addi >>> r1,r1,112                        | >>> |   0x944 <._fini+16>               ld >>> r0,16(r1)                        | >>> |   0x948 <._fini+20>               mtlr >>> r0                               | >>> |   0x94c <._fini+24> blr                                      | >>> |   0x950 <_IO_stdin_used>          .long >>> 0x20001                            | >>> |   0x954                           .long >>> 0x11b033b                          | >>> |   0x958                           .long >>> 0x18                               | >>> |   0x95c                           .long >>> 0x2                                | >>> |   0x960                           .long >>> 0xffffff60                         | >>> |   0x964                           .long >>> 0x48                               | >>> |   0x968                           .long >>> 0xffffff9c                         | >>> |   0x96c                           .long >>> 0x30                               | >>> +----------------------------------------------------------------------------+ >>> >>> exec No process (asm) In:                                        >>> L??   PC: ?? >>> (gdb) >>> ... >>> >>> The view corresponds to this objdump -D output: >>> ... >>> Disassembly of section .text: >>> >>>    ... >>> >>> 000000000000091c <__libc_start_main@plt>: >>>   91c:   38 00 00 00     li      r0,0 >>>   920:   4b ff ff d0     b       8f0 <__glink_PLTresolve> >>> >>> 0000000000000924 <__gmon_start__@plt>: >>>   924:   38 00 00 01     li      r0,1 >>>   928:   4b ff ff c8     b       8f0 <__glink_PLTresolve> >>> >>> 000000000000092c <__cxa_finalize@plt>: >>>   92c:   38 00 00 02     li      r0,2 >>>   930:   4b ff ff c0     b       8f0 <__glink_PLTresolve> >>> >>> Disassembly of section .fini: >>> >>> 0000000000000934 <._fini>: >>>   934:   7c 08 02 a6     mflr    r0 >>>   938:   f8 01 00 10     std     r0,16(r1) >>>   93c:   f8 21 ff 91     stdu    r1,-112(r1) >>>   940:   38 21 00 70     addi    r1,r1,112 >>>   944:   e8 01 00 10     ld      r0,16(r1) >>>   948:   7c 08 03 a6     mtlr    r0 >>>   94c:   4e 80 00 20     blr >>> >>> Disassembly of section .rodata: >>> >>> 0000000000000950 <_IO_stdin_used>: >>>   950:   00 02 00 01     .long 0x20001 >>> >>> Disassembly of section .eh_frame_hdr: >>> >>> 0000000000000954 <__GNU_EH_FRAME_HDR>: >>>   954:   01 1b 03 3b     .long 0x11b033b >>>   958:   00 00 00 18     .long 0x18 >>>   95c:   00 00 00 02     .long 0x2 >>>   960:   ff ff ff 60     .long 0xffffff60 >>>   964:   00 00 00 48     .long 0x48 >>>   968:   ff ff ff 9c     .long 0xffffff9c >>>   96c:   00 00 00 30     .long 0x30 >>> ... >>> >>> ISTM that we should not be disassembling the .rodata section. >>> >>> My first thought was to require tui_disassemble to stay in the same >>> section, >>> but after thinking about it a bit more decided that that was too >>> restrictive, >>> and instead went for requiring that we stay in code sections. >>> >>> So, I came up with this fix: >>> ... >>>        struct obj_section *section = find_pc_section (pc); >>>        if (section != nullptr >>>       && (bfd_section_flags (section->the_bfd_section) & SEC_CODE) >>> == 0) >>>     return pc; >>> ... >>> >>> Then I stumbled on PR tui/34399, which reports not being able to >>> scroll back >>> up after scrolling down.  The problem there is that we disassemble >>> code from a >>> section hole (0x401034-0x402000): >>> ... >>> Section Headers: >>>    [Nr] Name              Type             Address Offset >>>         Size              EntSize          Flags  Link  Info Align >>>    [ 0]                   NULL             0000000000000000 00000000 >>>         0000000000000000  0000000000000000           0 0     0 >>>    [ 1] .note.gnu.pr[...] NOTE             0000000000400190 00000190 >>>         0000000000000030  0000000000000000   A       0 0     8 >>>    [ 2] .note.gnu.bu[...] NOTE             00000000004001c0 000001c0 >>>         0000000000000024  0000000000000000   A       0 0     4 >>>    [ 3] .text             PROGBITS         0000000000401000 00001000 >>>         0000000000000034  0000000000000000  AX       0 0     1 >>>    [ 4] .rodata           PROGBITS         0000000000402000 00002000 >>>         0000000000000006  0000000000000000   A       0 0     1 >>>    [ 5] .symtab           SYMTAB           0000000000000000 00002008 >>>         0000000000000078  0000000000000018           6 1     8 >>>    [ 6] .strtab           STRTAB           0000000000000000 00002080 >>>         0000000000000019  0000000000000000           0 0     1 >>>    [ 7] .shstrtab         STRTAB           0000000000000000 00002099 >>>         000000000000004f  0000000000000000           0 0     1 >>> Key to Flags: >>>    W (write), A (alloc), X (execute), M (merge), S (strings), I (info), >>>    L (link order), O (extra OS processing required), G (group), T >>> (TLS), >>>    C (compressed), x (unknown), o (OS specific), E (exclude), >>>    D (mbind), l (large), p (processor specific) >>> ... >>> and once we call tui_find_backward_disassembly_start_address with an >>> address >>> ADDR for which find_pc_section (ADDR - 1) == nullptr, we are stuck >>> in the >>> section hole. >>> >>> Fix this by not disassembling section holes. >>> >>> This is achieved by slightly modifying the earlier fix: >>> ... >>>        if (section == nullptr >>>       || (bfd_section_flags (section->the_bfd_section) & SEC_CODE) >>> == 0) >>>     return pc; >>> ... >>> >>> Tested on x86_64-linux and ppc64-linux. >>> >>> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=34399 >>> --- >> >> Hi! >> >> Thank you for working on this! >> > > Hi Guinevere, > > thanks for the review. > >> I think your fix makes perfect sense, but I noticed an inaccuracy in >> the function comment. It predates your change, but I think now would >> be a reasonable time to fix it. The comment reads: >> >>      Function to disassemble up to COUNT instructions starting from >> address PC >>      into the ASM_LINES vector (which will be emptied of any previous >> contents). >>      Return the address of the COUNT'th instruction after pc. >> >> Considering that returning early is possible, I think this comment >> should say that we return the address right after the last >> disassembled instruction, or something to that effect. >> > > Agreed.  I've added a patch in a v2 ( > https://sourceware.org/pipermail/gdb-patches/2026-September/230205.html > ). > >> Also, I tried to reproduce this issue in x86 but couldn't. It would >> be nice if there was a test that exercised this, but it isn't a >> requirement for me. > > As for reproducing, initially I worked with the exec reported in the > PR, but Andrew's comment in his review made me realize I could use the > disassemble command to update the asm window. > > Then I managed to reproduce it quite easily using a hello world exec: > ... > $ gcc -g hello.c > ... > > The steps are: > - find a section hole using readelf -S a.out.  I found one in between >   .init and .plt. > - use the disassemble command to disassemble the section before the >   section hole.  In my case: "disassemble _init" > - scroll forward until .init is no longer visible (and maybe one >   instruction more) > - try to scroll back into _init > > I used this command for most of the setup: > ... > $ gdb -q -iex "set debuginfod enabled off" a.out -ex start -ex "layout > asm" -ex "disassemble _init" > ... > > Note that there's a start in there.  If that's not there, then we > can't scroll forward through the section hole to begin with. > > Indeed IWBN if there was a test-case, but I'm not sure how to build a > non-fragile architecture-neutral assembly file that is sure to have a > section hole. My idea was to have a .S with a code section and then a non-code section, to ensure that the non-code wouldn't be disassembled. But with Andrew's review I see that that should be changed so it makes sense to not have the test. > > Thanks, > - Tom > -- Cheers, Guinevere Larsen it/its she/her (deprecated)