Re: [PATCH] perf symbols: fix symbol offset breakage with separated debug info

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On Wed, Apr 25, 2012 at 11:41:30AM +0530, Prashanth Nageshappa wrote:
> perf resolves symbols to wrong offsets when debug info is separated
> from the lib/executable.
> 
> This patch is based on Dave Martin's initial work first published at
> http://lists.linaro.org/pipermail/linaro-dev/2010-August/000421.html
> 
> This patch loads the ELF section headers from a separate file if
> necessary, to avoid getting confused by the different section file
> offsets seen in debug images.  Invalid section headers are detected by
> checking for the presence of non-writable SHT_NOBITS sections, which
> don't make sense under normal circumstances.
> 
> In particular, this allows symbols in ET_EXEC images to get fixed up
> correctly in the presence of separated debug images.
> 
> 
> Signed-off-by: Prashanth Nageshappa <prashanth@xxxxxxxxxxxxxxxxxx>
> Cc: Dave Martin <dave.martin@xxxxxxxxxx>
> ---

Apologies for not responding earlier -- looks like your original mail
arrivied while I was on holiday.


There was an updated version of this patch, which responded to some
comments from Arnaldo, such as removing assert() calls and so on.
You should bring in the delta from that, but I don't think the
differences were huge:

https://lkml.org/lkml/2010/11/22/137

It looks like there were no comments on that post, and I was overtaken
by other work before anyone merged it.


Am I right in assuming that you haven't made significant changes other
than to rebase the patch onto current mainline?

Cheers
---Dave

> 
> 
>  tools/perf/util/symbol.c |  163 +++++++++++++++++++++++++++++++++++++++++++++-
>  1 files changed, 159 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
> index caaf75a..d32db9a 100644
> --- a/tools/perf/util/symbol.c
> +++ b/tools/perf/util/symbol.c
> @@ -1,3 +1,4 @@
> +#include <assert.h>
>  #include <dirent.h>
>  #include <errno.h>
>  #include <stdlib.h>
> @@ -1158,8 +1159,107 @@ static size_t elf_addr_to_index(Elf *elf, GElf_Addr addr)
>  	return -1;
>  }
>  
> +/**
> + * Read all section headers, copying them into a separate array so they survive
> + * elf_end.
> + *
> + * @elf: the libelf instance to operate on.
> + * @ehdr: the elf header: this must already have been read with gelf_getehdr().
> + * @count: the number of headers read is assigned to *count on successful
> + *	return.  count must not be NULL.
> + *
> + * Returns a pointer to the allocated headers, which should be deallocated with
> + * free() when no longer needed.
> + */
> +static GElf_Shdr *elf_get_all_shdrs(Elf *elf, GElf_Ehdr const *ehdr,
> +				    unsigned *count)
> +{
> +	GElf_Shdr *shdrs;
> +	Elf_Scn *scn;
> +	unsigned max_index = 0;
> +	unsigned i;
> +
> +	shdrs = malloc(ehdr->e_shnum * sizeof *shdrs);
> +	if (!shdrs)
> +		return NULL;
> +
> +	for (i = 0; i < ehdr->e_shnum; i++)
> +		shdrs[i].sh_type = SHT_NULL;
> +
> +	for (scn = NULL; (scn = elf_nextscn(elf, scn)); ) {
> +		size_t j;
> +
> +		/*
> +		 * Just assuming we get section 0, 1, ... in sequence may lead
> +		 * to wrong section indices.  Check the index explicitly:
> +		 */
> +		j = elf_ndxscn(scn);
> +		assert(j < ehdr->e_shnum);
> +
> +		if (j > max_index)
> +			max_index = j;
> +
> +		if (!gelf_getshdr(scn, &shdrs[j]))
> +			goto error;
> +	}
> +
> +	*count = max_index + 1;
> +	return shdrs;
> +
> +error:
> +	free(shdrs);
> +	return NULL;
> +}
> +
> +/**
> + * Check that the section headers @shdrs reflect accurately the file data
> + * layout of the image that was loaded during perf record.  This is generally
> + * not true for separated debug images generated with e.g.,
> + * objcopy --only-keep-debug.
> + *
> + * We identify invalid files by checking for non-empty sections which are
> + * declared as having no file data (SHT_NOBITS) but are not writable.
> + *
> + * @shdrs: the full set of section headers, as loaded by elf_get_all_shdrs().
> + * @count: the number of headers present in @shdrs.
> + *
> + * Returns 1 for valid headers, 0 otherwise.
> + */
> +static int elf_check_shdrs_valid(GElf_Shdr const *shdrs, unsigned count)
> +{
> +	unsigned i;
> +
> +	for (i = 0; i < count; i++) {
> +		if (shdrs[i].sh_type == SHT_NOBITS &&
> +		    !(shdrs[i].sh_flags & SHF_WRITE) &&
> +		    shdrs[i].sh_size != 0)
> +			return 0;
> +	}
> +
> +	return 1;
> +}
> +
> +/*
> + * Notes:
> + *
> + * If saved_shdrs is non-NULL, the section headers will be read if found, and
> + * will be used for address fixups.  saved_shdrs_count must also be non-NULL in
> + * this case.  This may be needed for separated debug images, since the section
> + * headers and symbols may need to come from separate images in that case.
> + *
> + * Note: irrespective of whether this function returns successfully,
> + * *saved_shdrs may get initialised if saved_shdrs is non-NULL.  It is the
> + * caller's responsibility to free() it when non longer needed.
> + *
> + * If want_symtab == 1, this function will only load symbols from .symtab
> + * sections.  Otherwise (want_symtab == 0), .dynsym or .symtab symbols are
> + * loaded.  This feature is used by dso__load() to search for the best image
> + * to load.
> + */
> +
>  static int dso__load_sym(struct dso *dso, struct map *map, const char *name,
>  			 int fd, symbol_filter_t filter, int kmodule,
> +			 GElf_Shdr **saved_shdrs, unsigned *saved_shdrs_count,
>  			 int want_symtab)
>  {
>  	struct kmap *kmap = dso->kernel ? map__kmap(map) : NULL;
> @@ -1178,6 +1278,9 @@ static int dso__load_sym(struct dso *dso, struct map *map, const char *name,
>  	int nr = 0;
>  	size_t opdidx = 0;
>  
> +	if (saved_shdrs != NULL)
> +		assert(saved_shdrs_count != NULL);
> +
>  	elf = elf_begin(fd, PERF_ELF_C_READ_MMAP, NULL);
>  	if (elf == NULL) {
>  		pr_debug("%s: cannot read %s ELF file.\n", __func__, name);
> @@ -1200,6 +1303,36 @@ static int dso__load_sym(struct dso *dso, struct map *map, const char *name,
>  			goto out_elf_end;
>  	}
>  
> +	/*
> +	 * Copy all section headers from the image if requested and if not
> +	 * already loaded.
> +	 */
> +	if (saved_shdrs != NULL && *saved_shdrs == NULL) {
> +		GElf_Shdr *shdrs;
> +		unsigned count;
> +
> +		shdrs = elf_get_all_shdrs(elf, &ehdr, &count);
> +		if (shdrs == NULL)
> +			goto out_elf_end;
> +
> +		/*
> +		 * Only keep the headers if they reflect the actual run-time
> +		 * image's file layout:
> +		 */
> +		if (elf_check_shdrs_valid(shdrs, count)) {
> +			*saved_shdrs = shdrs;
> +			*saved_shdrs_count = count;
> +		} else
> +			free(shdrs);
> +	}
> +
> +	/*
> +	 * If no genuine ELF headers are available yet, give up: we can't
> +	 * adjust symbols correctly in that case:
> +	 */
> +	if (saved_shdrs != NULL && *saved_shdrs == NULL)
> +		goto out_elf_end;
> +
>  	sec = elf_section_by_name(elf, &ehdr, &shdr, ".symtab", NULL);
>  	if (sec == NULL) {
>  		if (want_symtab)
> @@ -1344,12 +1477,25 @@ static int dso__load_sym(struct dso *dso, struct map *map, const char *name,
>  			goto new_symbol;
>  		}
>  
> +		/*
> +		 * Currently, symbols for shared objects and PIE executables
> +		 * (i.e., ET_DYN) do not seem to get adjusted.  This might need
> +		 * to change if file offset == virtual address is not actually
> +		 * guaranteed for these images.  ELF doesn't provide this
> +		 * guarantee natively.
> +		 */
>  		if (curr_dso->adjust_symbols) {
>  			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;
> +			if (saved_shdrs && *saved_shdrs &&
> +			    sym.st_shndx < *saved_shdrs_count)
> +				sym.st_value -=
> +					(*saved_shdrs)[sym.st_shndx].sh_addr -
> +					(*saved_shdrs)[sym.st_shndx].sh_offset;
> +			else
> +				sym.st_value -= shdr.sh_addr - shdr.sh_offset;
>  		}
>  		/*
>  		 * We need to figure out if the object was created from C++ sources
> @@ -1590,6 +1736,8 @@ int dso__load(struct dso *dso, struct map *map, symbol_filter_t filter)
>  	struct machine *machine;
>  	const char *root_dir;
>  	int want_symtab;
> +	GElf_Shdr *saved_shdrs = NULL;
> +	unsigned saved_shdrs_count;
>  
>  	dso__set_loaded(dso, map->type);
>  
> @@ -1692,6 +1840,7 @@ restart:
>  			continue;
>  
>  		ret = dso__load_sym(dso, map, name, fd, filter, 0,
> +				    &saved_shdrs, &saved_shdrs_count,
>  				    want_symtab);
>  		close(fd);
>  
> @@ -1713,14 +1862,19 @@ restart:
>  
>  	/*
>  	 * If we wanted a full symtab but no image had one,
> -	 * relax our requirements and repeat the search.
> +	 * relax our requirements and repeat the search,
> +	 * provided we saw some valid section headers:
>  	 */
> -	if (ret <= 0 && want_symtab) {
> +	if (ret <= 0 && want_symtab && saved_shdrs != NULL) {
>  		want_symtab = 0;
>  		goto restart;
>  	}
>  
>  	free(name);
> +
> +	if (saved_shdrs)
> +		free(saved_shdrs);
> +
>  	if (ret < 0 && strstr(dso->name, " (deleted)") != NULL)
>  		return 0;
>  	return ret;
> @@ -1989,7 +2143,8 @@ int dso__load_vmlinux(struct dso *dso, struct map *map,
>  
>  	dso__set_long_name(dso, (char *)vmlinux);
>  	dso__set_loaded(dso, map->type);
> -	err = dso__load_sym(dso, map, symfs_vmlinux, fd, filter, 0, 0);
> +	err = dso__load_sym(dso, map, symfs_vmlinux, fd, filter, 0,
> +			    NULL, NULL, 0);
>  	close(fd);
>  
>  	if (err > 0)
> 
> 

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