Message-ID: <b0a66e24-29c1-45e6-1578-5302127a926e@intel.com>
Date:   Tue, 31 Jan 2023 12:17:56 +0200
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Subject: Re: [PATCH 9/9] perf symbols: Get symbols for .plt.got for x86-64
Content-Language: en-US
To:     Namhyung Kim <namhyung@kernel.org>
Cc:     Arnaldo Carvalho de Melo <acme@kernel.org>,
        Jiri Olsa <jolsa@redhat.com>, Ian Rogers <irogers@google.com>,
        linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org
References: <20230127170222.9895-1-adrian.hunter@intel.com>
 <20230127170222.9895-10-adrian.hunter@intel.com>
 <Y9hSCR6WvpI4b5Cm@google.com>
From:   Adrian Hunter <adrian.hunter@intel.com>
Organization: Intel Finland Oy, Registered Address: PL 281, 00181 Helsinki,
 Business Identity Code: 0357606 - 4, Domiciled in Helsinki
In-Reply-To: <Y9hSCR6WvpI4b5Cm@google.com>
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Precedence: bulk

On 31/01/23 01:26, Namhyung Kim wrote:
> On Fri, Jan 27, 2023 at 07:02:22PM +0200, Adrian Hunter wrote:
>> For x86_64, determine a symbol for .plt.got entries. That requires
>> computing the target offset and finding that in .rela.dyn, which in
>> turn means .rela.dyn needs to be sorted by offset.
>>
>> Example:
>>
>>   In this example, the GNU C Library is using .plt.got for malloc and
>>   free.
>>
>>   Before:
>>
>>     $ 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.
>>     $ perf record -e intel_pt//u uname
>>     Linux
>>     [ perf record: Woken up 1 times to write data ]
>>     [ perf record: Captured and wrote 0.027 MB perf.data ]
>>     $ perf script --itrace=be --ns -F-event,+addr,-period,-comm,-tid,-cpu > /tmp/cmp1.txt
>>
>>   After:
>>
>>     $ perf script --itrace=be --ns -F-event,+addr,-period,-comm,-tid,-cpu > /tmp/cmp2.txt
>>     $ diff /tmp/cmp1.txt /tmp/cmp2.txt | head -12
>>     15509,15510c15509,15510
>>     < 27046.755390907:      7f0b2943e3ab _nl_normalize_codeset+0x5b (/usr/lib/x86_64-linux-gnu/libc.so.6) =>     7f0b29428380 offset_0x28380@plt+0x0 (/usr/lib/x86_64-linux-gnu/libc.so.6)
>>     < 27046.755390907:      7f0b29428384 offset_0x28380@plt+0x4 (/usr/lib/x86_64-linux-gnu/libc.so.6) =>     7f0b294a5120 malloc+0x0 (/usr/lib/x86_64-linux-gnu/libc.so.6)
>>     ---
>>     > 27046.755390907:      7f0b2943e3ab _nl_normalize_codeset+0x5b (/usr/lib/x86_64-linux-gnu/libc.so.6) =>     7f0b29428380 malloc@plt+0x0 (/usr/lib/x86_64-linux-gnu/libc.so.6)
>>     > 27046.755390907:      7f0b29428384 malloc@plt+0x4 (/usr/lib/x86_64-linux-gnu/libc.so.6) =>     7f0b294a5120 malloc+0x0 (/usr/lib/x86_64-linux-gnu/libc.so.6)
>>     15821,15822c15821,15822
>>     < 27046.755394865:      7f0b2943850c _nl_load_locale_from_archive+0x5bc (/usr/lib/x86_64-linux-gnu/libc.so.6) =>     7f0b29428370 offset_0x28370@plt+0x0 (/usr/lib/x86_64-linux-gnu/libc.so.6)
>>     < 27046.755394865:      7f0b29428374 offset_0x28370@plt+0x4 (/usr/lib/x86_64-linux-gnu/libc.so.6) =>     7f0b294a5460 cfree@GLIBC_2.2.5+0x0 (/usr/lib/x86_64-linux-gnu/libc.so.6)
>>     ---
>>     > 27046.755394865:      7f0b2943850c _nl_load_locale_from_archive+0x5bc (/usr/lib/x86_64-linux-gnu/libc.so.6) =>     7f0b29428370 free@plt+0x0 (/usr/lib/x86_64-linux-gnu/libc.so.6)
>>     > 27046.755394865:      7f0b29428374 free@plt+0x4 (/usr/lib/x86_64-linux-gnu/libc.so.6) =>     7f0b294a5460 cfree@GLIBC_2.2.5+0x0 (/usr/lib/x86_64-linux-gnu/libc.so.6)
>>
>> Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
>> ---
>>  tools/perf/util/symbol-elf.c | 158 ++++++++++++++++++++++++++++++++++-
>>  1 file changed, 154 insertions(+), 4 deletions(-)
>>
>> diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
>> index 254116d40e59..4fc8e7fc10f4 100644
>> --- a/tools/perf/util/symbol-elf.c
>> +++ b/tools/perf/util/symbol-elf.c
>> @@ -466,28 +466,178 @@ static bool machine_is_x86(GElf_Half e_machine)
>>  	return e_machine == EM_386 || e_machine == EM_X86_64;
>>  }
>>  
>> +struct rela_dyn {
>> +	GElf_Addr	offset;
>> +	u32		sym_idx;
>> +};
>> +
>> +struct rela_dyn_info {
>> +	struct dso	*dso;
>> +	Elf_Data	*plt_got_data;
>> +	u32		nr_entries;
>> +	struct rela_dyn	*sorted;
>> +	Elf_Data	*dynsym_data;
>> +	Elf_Data	*dynstr_data;
>> +	Elf_Data	*rela_dyn_data;
>> +};
>> +
>> +static void exit_rela_dyn(struct rela_dyn_info *di)
>> +{
>> +	free(di->sorted);
>> +}
>> +
>> +static int cmp_offset(const void *a, const void *b)
>> +{
>> +	const struct rela_dyn *va = a;
>> +	const struct rela_dyn *vb = b;
>> +
>> +	return va->offset < vb->offset ? -1 : (va->offset > vb->offset ? 1 : 0);
>> +}
>> +
>> +static int sort_rela_dyn(struct rela_dyn_info *di)
>> +{
>> +	u32 i, n;
>> +
>> +	di->sorted = calloc(di->nr_entries, sizeof(di->sorted[0]));
>> +	if (!di->sorted)
>> +		return -1;
>> +
>> +	/* Get data for sorting: the offset and symbol index */
>> +	for (i = 0, n = 0; i < di->nr_entries; i++) {
>> +		GElf_Rela rela;
>> +		u32 sym_idx;
>> +
>> +		gelf_getrela(di->rela_dyn_data, i, &rela);
>> +		sym_idx = GELF_R_SYM(rela.r_info);
>> +		if (sym_idx) {
>> +			di->sorted[n].sym_idx = sym_idx;
>> +			di->sorted[n].offset = rela.r_offset;
>> +			n += 1;
>> +		}
>> +	}
>> +
>> +	/* Sort by offset */
>> +	di->nr_entries = n;
>> +	qsort(di->sorted, n, sizeof(di->sorted[0]), cmp_offset);
>> +
>> +	return 0;
>> +}
>> +
>> +static void get_rela_dyn_info(Elf *elf, GElf_Ehdr *ehdr, struct rela_dyn_info *di, Elf_Scn *scn)
>> +{
>> +	GElf_Shdr rela_dyn_shdr;
>> +	GElf_Shdr shdr;
>> +
>> +	di->plt_got_data = elf_getdata(scn, NULL);
>> +
>> +	scn = elf_section_by_name(elf, ehdr, &rela_dyn_shdr, ".rela.dyn", NULL);
>> +	if (!scn || !rela_dyn_shdr.sh_link || !rela_dyn_shdr.sh_entsize)
>> +		return;
>> +
>> +	di->nr_entries = rela_dyn_shdr.sh_size / rela_dyn_shdr.sh_entsize;
>> +	di->rela_dyn_data = elf_getdata(scn, NULL);
>> +
>> +	scn = elf_getscn(elf, rela_dyn_shdr.sh_link);
>> +	if (!scn || !gelf_getshdr(scn, &shdr) || !shdr.sh_link)
>> +		return;
>> +
>> +	di->dynsym_data = elf_getdata(scn, NULL);
>> +	di->dynstr_data = elf_getdata(elf_getscn(elf, shdr.sh_link), NULL);
>> +
>> +	if (!di->plt_got_data || !di->dynstr_data || !di->dynsym_data || !di->rela_dyn_data)
>> +		return;
>> +
>> +	/* Sort into offset order */
>> +	sort_rela_dyn(di);
>> +}
>> +
>> +/* Get instruction displacement from a plt entry for x86_64 */
>> +static u32 get_x86_64_plt_disp(const u8 *p)
>> +{
>> +	u8 endbr64[] = {0xf3, 0x0f, 0x1e, 0xfa};
>> +	int n = 0;
>> +
>> +	/* Skip endbr64 */
>> +	if (!memcmp(p, endbr64, sizeof(endbr64)))
>> +		n += sizeof(endbr64);
>> +	/* Skip bnd prefix */
>> +	if (p[n] == 0xf2)
>> +		n += 1;
>> +	/* jmp with 4-byte displacement */
>> +	if (p[n] == 0xff && p[n + 1] == 0x25) {
>> +		n += 2;
>> +		/* Also add offset from start of entry to end of instruction */
>> +		return n + 4 + le32toh(*(const u32 *)(p + n));
>> +	}
>> +	return 0;
>> +}
>> +
>> +static bool get_plt_got_name(GElf_Shdr *shdr, size_t i,
>> +			     struct rela_dyn_info *di,
>> +			     char *buf, size_t buf_sz)
>> +{
>> +	void *p = di->plt_got_data->d_buf + i;
>> +	u32 disp = get_x86_64_plt_disp(p);
>> +	struct rela_dyn vi, *vr;
>> +	const char *sym_name;
>> +	char *demangled;
>> +	GElf_Sym sym;
>> +
>> +	if (!di->sorted || !disp)
>> +		return false;
>> +
>> +	/* Compute target offset of the .plt.got entry */
>> +	vi.offset = shdr->sh_offset + di->plt_got_data->d_off + i + disp;
>> +
>> +	/* Find that offset in .rela.dyn (sorted by offset) */
>> +	vr = bsearch(&vi, di->sorted, di->nr_entries, sizeof(di->sorted[0]), cmp_offset);
>> +	if (!vr)
>> +		return false;
>> +
>> +	/* Get the associated symbol */
>> +	gelf_getsym(di->dynsym_data, vr->sym_idx, &sym);
>> +	sym_name = elf_sym__name(&sym, di->dynstr_data);
>> +	demangled = demangle_sym(di->dso, 0, sym_name);
>> +	if (demangled != NULL)
>> +		sym_name = demangled;
>> +
>> +	snprintf(buf, buf_sz, "%s@plt", sym_name);
>> +
>> +	free(demangled);
>> +
>> +	return *sym_name;
>> +}
>> +
>>  static int dso__synthesize_plt_got_symbols(struct dso *dso, Elf *elf,
>>  					   GElf_Ehdr *ehdr,
>>  					   char *buf, size_t buf_sz)
>>  {
>> +	struct rela_dyn_info di = { .dso = dso };
>>  	struct symbol *sym;
>>  	GElf_Shdr shdr;
>>  	Elf_Scn *scn;
>> +	int err = -1;
>>  	size_t i;
>>  
>>  	scn = elf_section_by_name(elf, ehdr, &shdr, ".plt.got", NULL);
>>  	if (!scn || !shdr.sh_entsize)
>>  		return 0;
>>  
>> +	if (ehdr->e_machine == EM_X86_64)
>> +		get_rela_dyn_info(elf, ehdr, &di, scn);
> 
> What about EM_386?  Now I'm seeing segfaults on 32 bit test programs
> with .plt.got section.

Thanks for looking at this.

Looks like a late change in get_plt_got_name() did not
get tested for 32-bit.  It should be checking di->sorted
first.  EM_386 is not supported by this logic.

I will send a V2

> 
> Thanks,
> Namhyung
> 
> 
>> +
>>  	for (i = 0; i < shdr.sh_size; i += shdr.sh_entsize) {
>> -		snprintf(buf, buf_sz, "offset_%#zx@plt", shdr.sh_offset + i);
>> +		if (!get_plt_got_name(&shdr, i, &di, buf, buf_sz))
>> +			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);
>>  		if (!sym)
>> -			return -1;
>> +			goto out;
>>  		symbols__insert(&dso->symbols, sym);
>>  	}
>> -
>> -	return 0;
>> +	err = 0;
>> +out:
>> +	exit_rela_dyn(&di);
>> +	return err;
>>  }
>>  
>>  /*
>> -- 
>> 2.34.1
>>
>>