[PATCH bpf-next v3] 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
faccessat with AT_EACCESS flag.

Signed-off-by: Jon Doron <jond@wiz.io>
---
 tools/lib/bpf/btf.c    | 2 +-
 tools/lib/bpf/libbpf.c | 6 +++---
 tools/lib/bpf/usdt.c   | 2 +-
 3 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index b4d9a96c3c1b..d88647da2c7f 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -4664,7 +4664,7 @@ struct btf *btf__load_vmlinux_btf(void)
 	for (i = 0; i < ARRAY_SIZE(locations); i++) {
 		snprintf(path, PATH_MAX, locations[i], buf.release);
 
-		if (access(path, R_OK))
+		if (faccessat(AT_FDCWD, path, R_OK, AT_EACCESS))
 			continue;
 
 		btf = btf__parse(path, NULL);
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index e691f08a297f..184ce1684dcd 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -884,7 +884,7 @@ __u32 get_kernel_version(void)
 	__u32 major, minor, patch;
 	struct utsname info;
 
-	if (access(ubuntu_kver_file, R_OK) == 0) {
+	if (faccessat(AT_FDCWD, ubuntu_kver_file, R_OK, AT_EACCESS) == 0) {
 		FILE *f;
 
 		f = fopen(ubuntu_kver_file, "r");
@@ -9904,7 +9904,7 @@ static bool use_debugfs(void)
 	static int has_debugfs = -1;
 
 	if (has_debugfs < 0)
-		has_debugfs = access(DEBUGFS, F_OK) == 0;
+		has_debugfs = faccessat(AT_FDCWD, DEBUGFS, F_OK, AT_EACCESS) == 0;
 
 	return has_debugfs == 1;
 }
@@ -10721,7 +10721,7 @@ 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 has required permissions */
-			if (access(result, perm) < 0)
+			if (faccessat(AT_FDCWD, result, perm, AT_EACCESS) < 0)
 				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..e83b497c2245 100644
--- a/tools/lib/bpf/usdt.c
+++ b/tools/lib/bpf/usdt.c
@@ -282,7 +282,7 @@ 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 = faccessat(AT_FDCWD, ref_ctr_sysfs_path, F_OK, AT_EACCESS) == 0;
 
 	return man;
 }
-- 
2.37.3