Re: [PATCH V2 8/9] perf symbols: Start adding support for .plt.got for x86
From: Arnaldo Carvalho de Melo
Date: Thu Feb 02 2023 - 14:41:37 EST
Em Tue, Jan 31, 2023 at 03:16:24PM +0200, Adrian Hunter escreveu:
> For x86, .plt.got is used, for example, when the address is taken of a
> dynamically linked function. Start adding support by synthesizing a
> symbol for each entry. A subsequent patch will attempt to get a better
> name for the symbol.
>
> Example:
>
> Before:
>
> $ cat tstpltlib.c
> void fn1(void) {}
> void fn2(void) {}
> void fn3(void) {}
> void fn4(void) {}
> $ cat tstpltgot.c
> void fn1(void);
> void fn2(void);
> void fn3(void);
> void fn4(void);
>
> void callfn(void (*fn)(void))
> {
> fn();
> }
>
> int main()
> {
> fn4();
> fn1();
> callfn(fn3);
> fn2();
> fn3();
> return 0;
> }
> $ gcc --version
> gcc (Ubuntu 11.3.0-1ubuntu1~22.04) 11.3.0
> Copyright (C) 2021 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions. There is NO
> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
> $ gcc -Wall -Wextra -shared -o libtstpltlib.so tstpltlib.c
> $ gcc -Wall -Wextra -o tstpltgot tstpltgot.c -L . -ltstpltlib -Wl,-rpath="$(pwd)"
> $ readelf -SW tstpltgot | grep 'Name\|plt\|dyn'
> [Nr] Name Type Address Off Size ES Flg Lk Inf Al
> [ 6] .dynsym DYNSYM 00000000000003d8 0003d8 0000f0 18 A 7 1 8
> [ 7] .dynstr STRTAB 00000000000004c8 0004c8 0000c6 00 A 0 0 1
> [10] .rela.dyn RELA 00000000000005d8 0005d8 0000d8 18 A 6 0 8
> [11] .rela.plt RELA 00000000000006b0 0006b0 000048 18 AI 6 24 8
> [13] .plt PROGBITS 0000000000001020 001020 000040 10 AX 0 0 16
> [14] .plt.got PROGBITS 0000000000001060 001060 000020 10 AX 0 0 16
> [15] .plt.sec PROGBITS 0000000000001080 001080 000030 10 AX 0 0 16
> [23] .dynamic DYNAMIC 0000000000003d90 002d90 000210 10 WA 7 0 8
> $ perf record -e intel_pt//u --filter 'filter main @ ./tstpltgot , filter callfn @ ./tstpltgot' ./tstpltgot
> [ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 0.011 MB perf.data ]
> $ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
> 28393.810326915: tr strt 0 [unknown] => 562350baa1b2 main+0x0
> 28393.810326915: tr end call 562350baa1ba main+0x8 => 562350baa090 fn4@plt+0x0
> 28393.810326917: tr strt 0 [unknown] => 562350baa1bf main+0xd
> 28393.810326917: tr end call 562350baa1bf main+0xd => 562350baa080 fn1@plt+0x0
> 28393.810326917: tr strt 0 [unknown] => 562350baa1c4 main+0x12
> 28393.810326917: call 562350baa1ce main+0x1c => 562350baa199 callfn+0x0
> 28393.810326917: tr end call 562350baa1ad callfn+0x14 => 7f607d36110f fn3+0x0
> 28393.810326922: tr strt 0 [unknown] => 562350baa1af callfn+0x16
> 28393.810326922: return 562350baa1b1 callfn+0x18 => 562350baa1d3 main+0x21
> 28393.810326922: tr end call 562350baa1d3 main+0x21 => 562350baa0a0 fn2@plt+0x0
> 28393.810326924: tr strt 0 [unknown] => 562350baa1d8 main+0x26
> 28393.810326924: tr end call 562350baa1d8 main+0x26 => 562350baa060 [unknown] <- call to fn3 via .plt.got
> 28393.810326925: tr strt 0 [unknown] => 562350baa1dd main+0x2b
> 28393.810326925: tr end return 562350baa1e3 main+0x31 => 7f607d029d90 __libc_start_call_main+0x80
>
> After:
>
> $ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
> 28393.810326915: tr strt 0 [unknown] => 562350baa1b2 main+0x0
> 28393.810326915: tr end call 562350baa1ba main+0x8 => 562350baa090 fn4@plt+0x0
> 28393.810326917: tr strt 0 [unknown] => 562350baa1bf main+0xd
> 28393.810326917: tr end call 562350baa1bf main+0xd => 562350baa080 fn1@plt+0x0
> 28393.810326917: tr strt 0 [unknown] => 562350baa1c4 main+0x12
> 28393.810326917: call 562350baa1ce main+0x1c => 562350baa199 callfn+0x0
> 28393.810326917: tr end call 562350baa1ad callfn+0x14 => 7f607d36110f fn3+0x0
> 28393.810326922: tr strt 0 [unknown] => 562350baa1af callfn+0x16
> 28393.810326922: return 562350baa1b1 callfn+0x18 => 562350baa1d3 main+0x21
> 28393.810326922: tr end call 562350baa1d3 main+0x21 => 562350baa0a0 fn2@plt+0x0
> 28393.810326924: tr strt 0 [unknown] => 562350baa1d8 main+0x26
> 28393.810326924: tr end call 562350baa1d8 main+0x26 => 562350baa060 offset_0x1060@plt+0x0
> 28393.810326925: tr strt 0 [unknown] => 562350baa1dd main+0x2b
> 28393.810326925: tr end return 562350baa1e3 main+0x31 => 7f607d029d90 __libc_start_call_main+0x80
>
> Signed-off-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>
> ---
> tools/perf/util/symbol-elf.c | 29 +++++++++++++++++++++++++++++
> 1 file changed, 29 insertions(+)
>
> diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
> index 9e265a726418..254116d40e59 100644
> --- a/tools/perf/util/symbol-elf.c
> +++ b/tools/perf/util/symbol-elf.c
> @@ -466,6 +466,30 @@ static bool machine_is_x86(GElf_Half e_machine)
> return e_machine == EM_386 || e_machine == EM_X86_64;
> }
>
> +static int dso__synthesize_plt_got_symbols(struct dso *dso, Elf *elf,
> + GElf_Ehdr *ehdr,
> + char *buf, size_t buf_sz)
> +{
> + struct symbol *sym;
> + GElf_Shdr shdr;
> + Elf_Scn *scn;
> + size_t i;
> +
> + scn = elf_section_by_name(elf, ehdr, &shdr, ".plt.got", NULL);
> + if (!scn || !shdr.sh_entsize)
> + return 0;
> +
> + for (i = 0; i < shdr.sh_size; i += shdr.sh_entsize) {
> + snprintf(buf, buf_sz, "offset_%#zx@plt", shdr.sh_offset + i);
> + sym = symbol__new(shdr.sh_offset + i, shdr.sh_entsize, STB_GLOBAL, STT_FUNC, buf);
Had to apply the patch below with the following committer note:
Committer notes:
Fix build on 32-bit a arches where shdr.sh_offset is a uint32_t and on
64-bit whete it is uint64_t, so cast use PRIx64 and cast it to the
64-bit type, as done elsewhere in this same file.
diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
index b149cfea73d0f422..41882ae8452e5091 100644
--- a/tools/perf/util/symbol-elf.c
+++ b/tools/perf/util/symbol-elf.c
@@ -631,7 +631,7 @@ static int dso__synthesize_plt_got_symbols(struct dso *dso, Elf *elf,
for (i = 0; i < shdr.sh_size; i += shdr.sh_entsize) {
if (!get_plt_got_name(&shdr, i, &di, buf, buf_sz))
- snprintf(buf, buf_sz, "offset_%#zx@plt", shdr.sh_offset + i);
+ snprintf(buf, buf_sz, "offset_%#" PRIx64 "@plt", (u64)shdr.sh_offset + i);
sym = symbol__new(shdr.sh_offset + i, shdr.sh_entsize, STB_GLOBAL, STT_FUNC, buf);
if (!sym)
goto out;
> + if (!sym)
> + return -1;
> + symbols__insert(&dso->symbols, sym);
> + }
> +
> + return 0;
> +}
> +
> /*
> * We need to check if we have a .dynsym, so that we can handle the
> * .plt, synthesizing its symbols, that aren't on the symtabs (be it
> @@ -514,6 +538,11 @@ int dso__synthesize_plt_symbols(struct dso *dso, struct symsrc *ss)
> goto out_elf_end;
> symbols__insert(&dso->symbols, plt_sym);
>
> + /* Only x86 has .plt.got */
> + if (machine_is_x86(ehdr.e_machine) &&
> + dso__synthesize_plt_got_symbols(dso, elf, &ehdr, sympltname, sizeof(sympltname)))
> + goto out_elf_end;
> +
> /* Only x86 has .plt.sec */
> if (machine_is_x86(ehdr.e_machine) &&
> elf_section_by_name(elf, &ehdr, &plt_sec_shdr, ".plt.sec", NULL)) {
> --
> 2.34.1
>
--
- Arnaldo