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Subject[PATCH perf/core v9 01/16] perf-symbol: Introduce filename__readable to check readability
Date
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.
---
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)))
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