Re: [PATCH v5 1/8] perf: support specify vdso path in cmdline

[duchangbin]

On Thu, Jul 18, 2024 at 08:02:16PM +0300, Adrian Hunter wrote:
> On 2/07/24 07:18, Changbin Du wrote:
> > The vdso dumped from process memory (in buildid-cache) lacks debugging
> > info. To annotate vdso symbols with source lines we need specify a
> > debugging version.
> > 
> > For x86, we can find them from your local build as
> > arch/x86/entry/vdso/vdso{32,64}.so.dbg. Or they may reside in
> > /lib/modules/<version>/vdso/vdso{32,64}.so on Ubuntu. But notice that
> > the buildid has to match.
> > 
> > $ sudo perf record -a
> > $ sudo perf report --objdump=llvm-objdump \
> >   --vdso arch/x86/entry/vdso/vdso64.so.dbg,arch/x86/entry/vdso/vdso32.so.dbg
> > 
> > Samples: 17K of event 'cycles:P', 4000 Hz, Event count (approx.): 1760
> > __vdso_clock_gettime  /work/linux-host/arch/x86/entry/vdso/vdso64.so.d
> > Percent│       movq    -48(%rbp),%rsi
> >        │       testq   %rax,%rax
> >        │     ;               return vread_hvclock();
> >        │       movq    %rax,%rdx
> >        │     ;               if (unlikely(!vdso_cycles_ok(cycles)))
> >        │     ↑ js      eb
> >        │     ↑ jmp     74
> >        │     ;               ts->tv_sec = vdso_ts->sec;
> >   0.02 │147:   leaq    2(%rbx),%rax
> >        │       shlq    $4, %rax
> >        │       addq    %r10,%rax
> >        │     ;               while ((seq = READ_ONCE(vd->seq)) & 1) {
> >   9.38 │152:   movl    (%r10),%ecx
> > 
> > When doing cross platform analysis, we also need specify the vdso path if
> > we are interested in its symbols.
> > 
> 
> Just realized this is about objdump.  Sorry for not getting that
> earlier.
> 
> Like perf tools, objdump follows the paradigm of attempting to
> locate and use debug info transparently.  However, objdump looks
> at the installed debug info in /usr/lib/debug/.build-id/
> 
> For example, if the debug file is copied (or linked) there, then
> objdump or llvm-objdump will find it:
> 
> $ llvm-objdump-18 -dS ~/.debug/\[vdso\]/cf702469f4637840fd6ba1a8d8a628ff83253d04/vdso | head -20
> 
> ~/.debug/[vdso]/cf702469f4637840fd6ba1a8d8a628ff83253d04/vdso:      file format elf64-x86-64
> 
> Disassembly of section .text:
> 
> 00000000000006d0 <.text>:
>     6d0: 48 8d 3d 29 d9 ff ff          leaq    -0x26d7(%rip), %rdi     # 0xffffffffffffe000
>     6d7: eb 19                         jmp     0x6f2 <.text+0x22>
>     6d9: 4c 8b 0d 28 d9 ff ff          movq    -0x26d8(%rip), %r9      # 0xffffffffffffe008
>     6e0: 4c 8b 05 29 d9 ff ff          movq    -0x26d7(%rip), %r8      # 0xffffffffffffe010
>     6e7: 0f 01 f9                      rdtscp
>     6ea: 66 90                         nop
>     6ec: 8b 0f                         movl    (%rdi), %ecx
>     6ee: 39 ce                         cmpl    %ecx, %esi
>     6f0: 74 0e                         je      0x700 <.text+0x30>
>     6f2: 8b 37                         movl    (%rdi), %esi
>     6f4: 85 f6                         testl   %esi, %esi
>     6f6: 75 e1                         jne     0x6d9 <.text+0x9>
>     6f8: 48 c7 c0 ff ff ff ff          movq    $-0x1, %rax
>     6ff: c3                            retq
> $ sudo ln -s /lib/modules/6.9.2-local/build/arch/x86/entry/vdso/vdso64.so.dbg /usr/lib/debug/.build-id/cf/702469f4637840fd6ba1a8d8a628ff83253d04.debug
> $ llvm-objdump-18 -dS ~/.debug/\[vdso\]/cf702469f4637840fd6ba1a8d8a628ff83253d04/vdso | head -20                                                       
> 
> ~/.debug/[vdso]/cf702469f4637840fd6ba1a8d8a628ff83253d04/vdso:      file format elf64-x86-64
> 
> Disassembly of section .text:
> 
> 00000000000006d0 <vread_hvclock>:
> ; hv_read_tsc_page_tsc(const struct ms_hyperv_tsc_page *tsc_pg,
>     6d0: 48 8d 3d 29 d9 ff ff          leaq    -0x26d7(%rip), %rdi     # 0xffffffffffffe000 <hvclock_page>
>     6d7: eb 19                         jmp     0x6f2 <vread_hvclock+0x22>
> ;               scale = READ_ONCE(tsc_pg->tsc_scale);
>     6d9: 4c 8b 0d 28 d9 ff ff          movq    -0x26d8(%rip), %r9      # 0xffffffffffffe008 <hvclock_page+0x8>
> ;               offset = READ_ONCE(tsc_pg->tsc_offset);
>     6e0: 4c 8b 05 29 d9 ff ff          movq    -0x26d7(%rip), %r8      # 0xffffffffffffe010 <hvclock_page+0x10>
> ;       asm volatile(ALTERNATIVE_2("rdtsc",
>     6e7: 0f 31                         rdtsc
>     6e9: 90                            nop
>     6ea: 90                            nop
>     6eb: 90                            nop
> ;       } while (READ_ONCE(tsc_pg->tsc_sequence) != sequence);
>     6ec: 8b 0f                         movl    (%rdi), %ecx
> $
> 
> Would that solve your problem?
> 
> Notably, later versions of llvm-objdump have an option
> --debug-file-directory which makes it possible to
> have the debug info in a directory owned by the user,
> for example:
> 
> $ sudo rm /usr/lib/debug/.build-id/cf/702469f4637840fd6ba1a8d8a628ff83253d04.debug
> $ llvm-objdump-18 -dS ~/.debug/\[vdso\]/cf702469f4637840fd6ba1a8d8a628ff83253d04/vdso | head -20
> 
> ~/.debug/[vdso]/cf702469f4637840fd6ba1a8d8a628ff83253d04/vdso:      file format elf64-x86-64
> 
> Disassembly of section .text:
> 
> 00000000000006d0 <.text>:
>     6d0: 48 8d 3d 29 d9 ff ff          leaq    -0x26d7(%rip), %rdi     # 0xffffffffffffe000
>     6d7: eb 19                         jmp     0x6f2 <.text+0x22>
>     6d9: 4c 8b 0d 28 d9 ff ff          movq    -0x26d8(%rip), %r9      # 0xffffffffffffe008
>     6e0: 4c 8b 05 29 d9 ff ff          movq    -0x26d7(%rip), %r8      # 0xffffffffffffe010
>     6e7: 0f 01 f9                      rdtscp
>     6ea: 66 90                         nop
>     6ec: 8b 0f                         movl    (%rdi), %ecx
>     6ee: 39 ce                         cmpl    %ecx, %esi
>     6f0: 74 0e                         je      0x700 <.text+0x30>
>     6f2: 8b 37                         movl    (%rdi), %esi
>     6f4: 85 f6                         testl   %esi, %esi
>     6f6: 75 e1                         jne     0x6d9 <.text+0x9>
>     6f8: 48 c7 c0 ff ff ff ff          movq    $-0x1, %rax
>     6ff: c3                            retq
> $ mkdir -p /tmp/debug/.build-id/cf/
> $ ln -s /lib/modules/6.9.2-local/build/arch/x86/entry/vdso/vdso64.so.dbg /tmp/debug/.build-id/cf/702469f4637840fd6ba1a8d8a628ff83253d04.debug        
> $ llvm-objdump-18 --debug-file-directory /tmp/debug -dS ~/.debug/\[vdso\]/cf702469f4637840fd6ba1a8d8a628ff83253d04/vdso | head -20
> 
> ~/.debug/[vdso]/cf702469f4637840fd6ba1a8d8a628ff83253d04/vdso:      file format elf64-x86-64
> 
> Disassembly of section .text:
> 
> 00000000000006d0 <vread_hvclock>:
> ; hv_read_tsc_page_tsc(const struct ms_hyperv_tsc_page *tsc_pg,
>     6d0: 48 8d 3d 29 d9 ff ff          leaq    -0x26d7(%rip), %rdi     # 0xffffffffffffe000 <hvclock_page>
>     6d7: eb 19                         jmp     0x6f2 <vread_hvclock+0x22>
> ;               scale = READ_ONCE(tsc_pg->tsc_scale);
>     6d9: 4c 8b 0d 28 d9 ff ff          movq    -0x26d8(%rip), %r9      # 0xffffffffffffe008 <hvclock_page+0x8>
> ;               offset = READ_ONCE(tsc_pg->tsc_offset);
>     6e0: 4c 8b 05 29 d9 ff ff          movq    -0x26d7(%rip), %r8      # 0xffffffffffffe010 <hvclock_page+0x10>
> ;       asm volatile(ALTERNATIVE_2("rdtsc",
>     6e7: 0f 31                         rdtsc
>     6e9: 90                            nop
>     6ea: 90                            nop
>     6eb: 90                            nop
> ;       } while (READ_ONCE(tsc_pg->tsc_sequence) != sequence);
>     6ec: 8b 0f                         movl    (%rdi), %ecx
> $
>
Yes, not only llvm-objdump. I can see other tools also can handle this and even
can download from online by Debuginfod.
  - perf can search debugging info there if HAVE_DEBUGINFOD_SUPPORT.
  - gdb can search debugging info there and download via Debuginfod if not exist.

While above requires debugging files are placed correctly in
/usr/lib/debug/.build-id/. For vdso mostly you need to copy it manully.

The target of this series is to handle it transparently, espacially for locally
built kernels.

> 
> 

-- 
Cheers,
Changbin Du