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Wed, 09 Sep 2026 11:27:18 +0000 Message-ID: Date: Wed, 9 Sep 2026 13:27:18 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] [gdb/tui] Don't disassemble non-code sections and section holes To: Andrew Burgess , gdb-patches@sourceware.org References: <20260819143514.2221499-1-tdevries@suse.de> <875x0lrpeo.fsf@redhat.com> Content-Language: en-US From: Tom de Vries In-Reply-To: <875x0lrpeo.fsf@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Spamd-Result: default: False [-4.30 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-0.996]; MIME_GOOD(-0.10)[text/plain]; RCVD_VIA_SMTP_AUTH(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; ARC_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; RCPT_COUNT_TWO(0.00)[2]; RCVD_TLS_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; URIBL_BLOCKED(0.00)[gdb.tui:url,suse.de:mid,suse.de:email,imap1.dmz-prg2.suse.org:helo,plt:email,sourceware.org:url]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; DBL_BLOCKED_OPENRESOLVER(0.00)[sourceware.org:url, plt:email, gdb.tui:url, suse.de:mid, suse.de:email, imap1.dmz-prg2.suse.org:helo] 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 9/4/26 5:45 PM, Andrew Burgess wrote: > Tom de Vries writes: > >> 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; >> ... > > TUI disassembly has been an ongoing issue for as long as I've been > contributing to GDB :-/ Getting the logic right for scrolling > forward/backward has always been an issue. > > I'm not sure this change is the right fix though. I have two concerns. > > In CLI mode, if I stop the inferior and then use `disassemble START,END` > syntax, then I can disassemble any range of memory, not just executable > sections. > > If I switch to TUI mode, without this patch and try the same disassemble > command then I'll get a screen full of instructions starting at START. Hi Andrew, thanks for the review. I see your point. I've submitted now a v2 that drops this restrictive approach, and instead tries to enable scrolling up and down as much as possible ( https://sourceware.org/pipermail/gdb-patches/2026-September/230207.html ). > Is that useful given these aren't code sections? I'm not really sure. > Yeah. I filed a related PR ( https://sourceware.org/bugzilla/show_bug.cgi?id=34604 ). > But if I apply your patch and try the same disassemble command then I > get a blank ASM window. > > There are for sure ways that a user can work around this, `x/i` will > dump the instructions to the CMD window, even in TUI mode. But the > blank window still looks wrong to me, and I think it is not what users > will expect. > Agreed. > The other problem I see is that this will, if I understand correctly, > prevent disassembly of memory that is not backed by an ELF section. For > example, a JIT compiling interpreter. Given such a tool I'd expect to > be able to disassemble the generated code regions, but these will likely > not be backed by an ELF section. Good point. I've made a note the JIT scenario in aforementioned PR. Thanks, - Tom