linux-kernel - Re: [PATCH perf/core v9 01/16] perf-symbol: Introduce filename_

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Date:	Mon, 30 May 2016 13:03:17 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Masami Hiramatsu <mhiramat@...nel.org>
Cc:	linux-kernel@...r.kernel.org, Namhyung Kim <namhyung@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Hemant Kumar <hemant@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>,
	Brendan Gregg <brendan.d.gregg@...il.com>
Subject: Re: [PATCH perf/core v9 01/16] perf-symbol: Introduce
 filename__readable to check readability

Em Sun, May 29, 2016 at 12:15:13AM +0900, Masami Hiramatsu escreveu:
> Introduce filename__readable to check readability by opening
> the file directly. Since the access(R_OK) just checks the
> readability based on real UID/GID, it is ignored that the
> effective UID/GID and capabilities for some special file (e.g.
> /proc/kcore).
> filename__readable() directly opens given file with O_RDONLY
> so that the kernel checks it by effective UID/GID and capabilities.


You missed the Signed-off-by line.

- Arnaldo

> ---
>  tools/perf/util/symbol.c |   32 ++++++++++++++++++++++----------
>  1 file changed, 22 insertions(+), 10 deletions(-)
> 
> diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
> index 54c4ff2..a469346 100644
> --- a/tools/perf/util/symbol.c
> +++ b/tools/perf/util/symbol.c
> @@ -1641,6 +1641,20 @@ static int find_matching_kcore(struct map *map, char *dir, size_t dir_sz)
>  	return ret;
>  }
>  
> +/*
> + * Use open(O_RDONLY) to check readability directly instead of access(R_OK)
> + * since access(R_OK) only checks with real UID/GID but open() use effective
> + * UID/GID and actual capabilities (e.g. /proc/kcore requires CAP_SYS_RAWIO).
> + */
> +static bool filename__readable(const char *file)
> +{
> +	int fd = open(file, O_RDONLY);
> +	if (fd < 0)
> +		return false;
> +	close(fd);
> +	return true;
> +}
> +
>  static char *dso__find_kallsyms(struct dso *dso, struct map *map)
>  {
>  	u8 host_build_id[BUILD_ID_SIZE];
> @@ -1668,7 +1682,6 @@ static char *dso__find_kallsyms(struct dso *dso, struct map *map)
>  	/* Use /proc/kallsyms if possible */
>  	if (is_host) {
>  		DIR *d;
> -		int fd;
>  
>  		/* If no cached kcore go with /proc/kallsyms */
>  		d = opendir(path);
> @@ -1677,16 +1690,15 @@ static char *dso__find_kallsyms(struct dso *dso, struct map *map)
>  		closedir(d);
>  
>  		/*
> -		 * Do not check the build-id cache, until we know we cannot use
> -		 * /proc/kcore.
> +		 * Do not check the build-id cache, unless we know we cannot use
> +		 * /proc/kcore or module maps don't match to /proc/kallsyms.
> +		 * To check readability of /proc/kcore, do not use access(R_OK)
> +		 * since /proc/kcore requires CAP_SYS_RAWIO to read and access
> +		 * can't check it.
>  		 */
> -		fd = open("/proc/kcore", O_RDONLY);
> -		if (fd != -1) {
> -			close(fd);
> -			/* If module maps match go with /proc/kallsyms */
> -			if (!validate_kcore_addresses("/proc/kallsyms", map))
> -				goto proc_kallsyms;
> -		}
> +		if (filename__readable("/proc/kcore") &&
> +		    !validate_kcore_addresses("/proc/kallsyms", map))
> +			goto proc_kallsyms;
>  
>  		/* Find kallsyms in build-id cache with kcore */
>  		if (!find_matching_kcore(map, path, sizeof(path)))