Re: [PATCH v3 1/2] perf symbol: Correct address for bss symbols

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Sun, Jul 24, 2022 at 02:00:12PM +0800, Leo Yan escreveu:
> When using 'perf mem' and 'perf c2c', an issue is observed that tool
> reports the wrong offset for global data symbols.  This is a common
> issue on both x86 and Arm64 platforms.
> 
> Let's see an example, for a test program, below is the disassembly for
> its .bss section which is dumped with objdump:
> 
>   ...
> 
>   Disassembly of section .bss:
> 
>   0000000000004040 <completed.0>:
>   	...
> 
>   0000000000004080 <buf1>:
>   	...
> 
>   00000000000040c0 <buf2>:
>   	...
> 
>   0000000000004100 <thread>:
>   	...
> 
> First we used 'perf mem record' to run the test program and then used
> 'perf --debug verbose=4 mem report' to observe what's the symbol info
> for 'buf1' and 'buf2' structures.
> 
>   # ./perf mem record -e ldlat-loads,ldlat-stores -- false_sharing.exe 8

Can you share the source code for your false sharing proggie? We need to
have something in 'perf test' exercising these routines :-)

>   # ./perf --debug verbose=4 mem report
>     ...
>     dso__load_sym_internal: adjusting symbol: st_value: 0x40c0 sh_addr: 0x4040 sh_offset: 0x3028
>     symbol__new: buf2 0x30a8-0x30e8
>     ...
>     dso__load_sym_internal: adjusting symbol: st_value: 0x4080 sh_addr: 0x4040 sh_offset: 0x3028
>     symbol__new: buf1 0x3068-0x30a8
>     ...
> 
> Perf tool relies on libelf to parse symbols, in executable and shared
> object files, 'st_value' holds a virtual address; 'sh_addr' is the
> address at which section's first byte should reside in memory, and
> 'sh_offset' is the byte offset from the beginning of the file to the
> first byte in the section.  Perf tool uses below formula to convert
> symbol's memory address to file address:
> 
>   file_address = st_value - sh_addr + sh_offset
>                     ^
>                     ` Memory address
> 
> We can see the final adjusted address ranges for buf1 and buf2 are
> [0x30a8-0x30e8) and [0x3068-0x30a8) respectively, apparently this is
> incorrect, in the code, the structure for 'buf1' and 'buf2' specifies
> compiler attribute with 64-byte alignment.
> 
> The problem happens for 'sh_offset', libelf returns it as 0x3028 which
> is not 64-byte aligned, combining with disassembly, it's likely libelf
> doesn't respect the alignment for .bss section, therefore, it doesn't
> return the aligned value for 'sh_offset'.
> 
> Suggested by Fangrui Song, elf file contains program header which
> contains PT_LOAD segments, the fields p_vaddr and p_offset in PT_LOAD
> segments contain the execution info.  A better choice for converting
> memory address to file address is using the formula:
> 
>   file_address = st_value - p_vaddr + p_offset
> 
> This patch introduces function elf_read_program_header() which returns
> out the program header based on the passed 'st_value', then it uses
> above formula to calculate the symbol file address; and the debugging
> log is updated respectively.
> 
> After applying the change:
> 
>   # ./perf --debug verbose=4 mem report
>     ...
>     dso__load_sym_internal: adjusting symbol: st_value: 0x40c0 p_vaddr: 0x3d28 p_offset: 0x2d28
>     symbol__new: buf2 0x30c0-0x3100
>     ...
>     dso__load_sym_internal: adjusting symbol: st_value: 0x4080 p_vaddr: 0x3d28 p_offset: 0x2d28
>     symbol__new: buf1 0x3080-0x30c0
>     ...
> 
> Fixes: f17e04afaff8 ("perf report: Fix ELF symbol parsing")
> Reported-by: Chang Rui <changruinj@gmail.com>
> Suggested-by: Fangrui Song <maskray@google.com>
> Signed-off-by: Leo Yan <leo.yan@linaro.org>
> ---
>  tools/perf/util/symbol-elf.c | 45 ++++++++++++++++++++++++++++++++----
>  1 file changed, 41 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
> index ecd377938eea..ef6ced5c5746 100644
> --- a/tools/perf/util/symbol-elf.c
> +++ b/tools/perf/util/symbol-elf.c
> @@ -233,6 +233,33 @@ Elf_Scn *elf_section_by_name(Elf *elf, GElf_Ehdr *ep,
>  	return NULL;
>  }
>  
> +static int elf_read_program_header(Elf *elf, u64 vaddr, GElf_Phdr *phdr)
> +{
> +	size_t i, phdrnum;
> +	u64 sz;
> +
> +	if (elf_getphdrnum(elf, &phdrnum))
> +		return -1;
> +
> +	for (i = 0; i < phdrnum; i++) {
> +		if (gelf_getphdr(elf, i, phdr) == NULL)
> +			return -1;
> +
> +		if (phdr->p_type != PT_LOAD)
> +			continue;
> +
> +		sz = max(phdr->p_memsz, phdr->p_filesz);
> +		if (!sz)
> +			continue;
> +
> +		if (vaddr >= phdr->p_vaddr && (vaddr < phdr->p_vaddr + sz))
> +			return 0;
> +	}
> +
> +	/* Not found any valid program header */
> +	return -1;
> +}
> +
>  static bool want_demangle(bool is_kernel_sym)
>  {
>  	return is_kernel_sym ? symbol_conf.demangle_kernel : symbol_conf.demangle;
> @@ -1209,6 +1236,7 @@ dso__load_sym_internal(struct dso *dso, struct map *map, struct symsrc *syms_ss,
>  					sym.st_value);
>  			used_opd = true;
>  		}
> +
>  		/*
>  		 * When loading symbols in a data mapping, ABS symbols (which
>  		 * has a value of SHN_ABS in its st_shndx) failed at
> @@ -1262,11 +1290,20 @@ dso__load_sym_internal(struct dso *dso, struct map *map, struct symsrc *syms_ss,
>  				goto out_elf_end;
>  		} else if ((used_opd && runtime_ss->adjust_symbols) ||
>  			   (!used_opd && syms_ss->adjust_symbols)) {
> +			GElf_Phdr phdr;
> +
> +			if (elf_read_program_header(syms_ss->elf,
> +						    (u64)sym.st_value, &phdr)) {
> +				pr_warning("%s: failed to find program header for "
> +					   "symbol: %s st_value: %#" PRIx64 "\n",
> +					   __func__, elf_name, (u64)sym.st_value);
> +				continue;
> +			}
>  			pr_debug4("%s: adjusting symbol: st_value: %#" PRIx64 " "
> -				  "sh_addr: %#" PRIx64 " sh_offset: %#" PRIx64 "\n", __func__,
> -				  (u64)sym.st_value, (u64)shdr.sh_addr,
> -				  (u64)shdr.sh_offset);
> -			sym.st_value -= shdr.sh_addr - shdr.sh_offset;
> +				  "p_vaddr: %#" PRIx64 " p_offset: %#" PRIx64 "\n",
> +				  __func__, (u64)sym.st_value, (u64)phdr.p_vaddr,
> +				  (u64)phdr.p_offset);
> +			sym.st_value -= phdr.p_vaddr - phdr.p_offset;
>  		}
>  
>  		demangled = demangle_sym(dso, kmodule, elf_name);
> -- 
> 2.25.1

-- 

- Arnaldo