[PATCH bpf-next v2] libbpf: Fix the case of running as non-root with capabilities

[Jon Doron <arilou@gmail.com>]

From: Jon Doron <jond@wiz.io>

When running rootless with special capabilities like:
FOWNER / DAC_OVERRIDE / DAC_READ_SEARCH

The "access" API will not make the proper check if there is really
access to a file or not.

From the access man page:
"
The check is done using the calling process's real UID and GID, rather
than the effective IDs as is done when actually attempting an operation
(e.g., open(2)) on the file.  Similarly, for the root user, the check
uses the set of permitted capabilities  rather than the set of effective
capabilities; ***and for non-root users, the check uses an empty set of
capabilities.***
"

What that means is that for non-root user the access API will not do the
proper validation if the process really has permission to a file or not.

To resolve this this patch replaces all the access API calls with stat.

Signed-off-by: Jon Doron <jond@wiz.io>
---
 tools/lib/bpf/btf.c    |  3 ++-
 tools/lib/bpf/libbpf.c | 11 ++++++++---
 tools/lib/bpf/usdt.c   |  4 +++-
 3 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index 2d14f1a52d7a..33ad4792d9e8 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -4663,13 +4663,14 @@ struct btf *btf__load_vmlinux_btf(void)
 	struct utsname buf;
 	struct btf *btf;
 	int i, err;
+	struct stat sb;
 
 	uname(&buf);
 
 	for (i = 0; i < ARRAY_SIZE(locations); i++) {
 		snprintf(path, PATH_MAX, locations[i].path_fmt, buf.release);
 
-		if (access(path, R_OK))
+		if (stat(path, &sb))
 			continue;
 
 		if (locations[i].raw_btf)
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 50d41815f431..c7fbce4225b5 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -875,8 +875,9 @@ __u32 get_kernel_version(void)
 	const char *ubuntu_kver_file = "/proc/version_signature";
 	__u32 major, minor, patch;
 	struct utsname info;
+	struct stat sb;
 
-	if (access(ubuntu_kver_file, R_OK) == 0) {
+	if (stat(ubuntu_kver_file, &sb) == 0) {
 		FILE *f;
 
 		f = fopen(ubuntu_kver_file, "r");
@@ -9877,9 +9878,10 @@ static int append_to_file(const char *file, const char *fmt, ...)
 static bool use_debugfs(void)
 {
 	static int has_debugfs = -1;
+	struct stat sb;
 
 	if (has_debugfs < 0)
-		has_debugfs = access(DEBUGFS, F_OK) == 0;
+		has_debugfs = stat(DEBUGFS, &sb) == 0;
 
 	return has_debugfs == 1;
 }
@@ -10681,6 +10683,7 @@ static int resolve_full_path(const char *file, char *result, size_t result_sz)
 		for (s = search_paths[i]; s != NULL; s = strchr(s, ':')) {
 			char *next_path;
 			int seg_len;
+			struct stat sb;
 
 			if (s[0] == ':')
 				s++;
@@ -10690,7 +10693,9 @@ static int resolve_full_path(const char *file, char *result, size_t result_sz)
 				continue;
 			snprintf(result, result_sz, "%.*s/%s", seg_len, s, file);
 			/* ensure it is an executable file/link */
-			if (access(result, R_OK | X_OK) < 0)
+			if (stat(result, &sb) < 0)
+				continue;
+			if ((sb.st_mode & (S_IROTH | S_IXOTH)) != (S_IROTH | S_IXOTH))
 				continue;
 			pr_debug("resolved '%s' to '%s'\n", file, result);
 			return 0;
diff --git a/tools/lib/bpf/usdt.c b/tools/lib/bpf/usdt.c
index d18e37982344..19a6fbcfe9c0 100644
--- a/tools/lib/bpf/usdt.c
+++ b/tools/lib/bpf/usdt.c
@@ -7,6 +7,7 @@
 #include <libelf.h>
 #include <gelf.h>
 #include <unistd.h>
+#include <sys/stat.h>
 #include <linux/ptrace.h>
 #include <linux/kernel.h>
 
@@ -257,6 +258,7 @@ struct usdt_manager *usdt_manager_new(struct bpf_object *obj)
 	static const char *ref_ctr_sysfs_path = "/sys/bus/event_source/devices/uprobe/format/ref_ctr_offset";
 	struct usdt_manager *man;
 	struct bpf_map *specs_map, *ip_to_spec_id_map;
+	struct stat sb;
 
 	specs_map = bpf_object__find_map_by_name(obj, "__bpf_usdt_specs");
 	ip_to_spec_id_map = bpf_object__find_map_by_name(obj, "__bpf_usdt_ip_to_spec_id");
@@ -282,7 +284,7 @@ struct usdt_manager *usdt_manager_new(struct bpf_object *obj)
 	 * If this is not supported, USDTs with semaphores will not be supported.
 	 * Added in: a6ca88b241d5 ("trace_uprobe: support reference counter in fd-based uprobe")
 	 */
-	man->has_sema_refcnt = access(ref_ctr_sysfs_path, F_OK) == 0;
+	man->has_sema_refcnt = stat(ref_ctr_sysfs_path, &sb) == 0;
 
 	return man;
 }
-- 
2.37.3