[PATCH v2] perf cap: Remove used_root parameter and simplify capability checks

[Ian Rogers <irogers@google.com>]

Refactor perf_cap__capable() to completely remove the used_root out-parameter
as requested by the maintainer. Relying on an explicit used_root boolean
poisoned sequential capability checks (e.g. failing CAP_SYS_ADMIN checks
poisoning the flag for subsequent CAP_PERFMON evaluations for unprivileged
users) and created redundant complexity across check_ftrace_capable(),
symbol__read_kptr_restrict(), and perf_event_paranoid_check().

Streamline the capability API to perform a pure true/false boolean
evaluation. The function checks the Effective set using SYS_capget; if
the syscall is missing or fails on legacy kernels, it cleanly falls back
to checking EUID == 0. This perfectly preserves modern capability-aware host
sessions, guarantees transparent fallback for older kernels, and correctly
rejects privileged operations for containerized root processes that have
explicitly dropped their capability bounding and permitted sets.

Fixes: e25ebda78e23 ("perf cap: Tidy up and improve capability testing")
Suggested-by: Namhyung Kim <namhyung@kernel.org>
Assisted-by: Antigravity:gemini-3.5-flash
Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/perf/builtin-ftrace.c  | 13 +++----------
 tools/perf/util/bpf-filter.c | 22 +++++++++-------------
 tools/perf/util/cap.c        |  4 +---
 tools/perf/util/cap.h        |  3 +--
 tools/perf/util/symbol.c     |  3 +--
 tools/perf/util/util.c       | 12 +++---------
 6 files changed, 18 insertions(+), 39 deletions(-)

diff --git a/tools/perf/builtin-ftrace.c b/tools/perf/builtin-ftrace.c
index 9e4c5220d43c..f7126196b092 100644
--- a/tools/perf/builtin-ftrace.c
+++ b/tools/perf/builtin-ftrace.c
@@ -72,18 +72,11 @@ static void ftrace__workload_exec_failed_signal(int signo __maybe_unused,
 
 static bool check_ftrace_capable(void)
 {
-	bool used_root;
-
-	if (perf_cap__capable(CAP_PERFMON, &used_root))
-		return true;
-
-	if (!used_root && perf_cap__capable(CAP_SYS_ADMIN, &used_root))
+	if (perf_cap__capable(CAP_PERFMON) ||
+	    perf_cap__capable(CAP_SYS_ADMIN))
 		return true;
 
-	pr_err("ftrace only works for %s!\n",
-		used_root ? "root"
-			  : "users with the CAP_PERFMON or CAP_SYS_ADMIN capability"
-		);
+	pr_err("ftrace only works for users with the CAP_PERFMON or CAP_SYS_ADMIN capability!\n");
 	return false;
 }
 
diff --git a/tools/perf/util/bpf-filter.c b/tools/perf/util/bpf-filter.c
index 1a2e7b388d57..bcd81084e342 100644
--- a/tools/perf/util/bpf-filter.c
+++ b/tools/perf/util/bpf-filter.c
@@ -629,24 +629,20 @@ struct perf_bpf_filter_expr *perf_bpf_filter_expr__new(enum perf_bpf_filter_term
 
 static bool check_bpf_filter_capable(void)
 {
-	bool used_root;
+	int fd;
 
-	if (perf_cap__capable(CAP_BPF, &used_root))
+	if (perf_cap__capable(CAP_BPF))
 		return true;
 
-	if (!used_root) {
-		/* Check if root already pinned the filter programs and maps */
-		int fd = get_pinned_fd("filters");
-
-		if (fd >= 0) {
-			close(fd);
-			return true;
-		}
+	/* Check if root already pinned the filter programs and maps */
+	fd = get_pinned_fd("filters");
+	if (fd >= 0) {
+		close(fd);
+		return true;
 	}
 
-	pr_err("Error: BPF filter only works for %s!\n"
-	       "\tPlease run 'perf record --setup-filter pin' as root first.\n",
-	       used_root ? "root" : "users with the CAP_BPF capability");
+	pr_err("Error: BPF filter only works for users with the CAP_BPF capability!\n"
+	       "\tPlease run 'perf record --setup-filter pin' as root first.\n");
 
 	return false;
 }
diff --git a/tools/perf/util/cap.c b/tools/perf/util/cap.c
index ac6d1d9a523d..272bd8255ff1 100644
--- a/tools/perf/util/cap.c
+++ b/tools/perf/util/cap.c
@@ -12,7 +12,7 @@
 
 #define MAX_LINUX_CAPABILITY_U32S _LINUX_CAPABILITY_U32S_3
 
-bool perf_cap__capable(int cap, bool *used_root)
+bool perf_cap__capable(int cap)
 {
 	struct __user_cap_header_struct header = {
 		.version = _LINUX_CAPABILITY_VERSION_3,
@@ -21,7 +21,6 @@ bool perf_cap__capable(int cap, bool *used_root)
 	struct __user_cap_data_struct data[MAX_LINUX_CAPABILITY_U32S] = {};
 	__u32 cap_val;
 
-	*used_root = false;
 	while (syscall(SYS_capget, &header, &data[0]) == -1) {
 		/* Retry, first attempt has set the header.version correctly. */
 		if (errno == EINVAL && header.version != _LINUX_CAPABILITY_VERSION_3 &&
@@ -29,7 +28,6 @@ bool perf_cap__capable(int cap, bool *used_root)
 			continue;
 
 		pr_debug2("capget syscall failed (%m) fall back on root check\n");
-		*used_root = true;
 		return geteuid() == 0;
 	}
 
diff --git a/tools/perf/util/cap.h b/tools/perf/util/cap.h
index c1b8ac033ccc..bf09fb20c779 100644
--- a/tools/perf/util/cap.h
+++ b/tools/perf/util/cap.h
@@ -18,7 +18,6 @@
 #define CAP_BPF		39
 #endif
 
-/* Query if a capability is supported, used_root is set if the fallback root check was used. */
-bool perf_cap__capable(int cap, bool *used_root);
+bool perf_cap__capable(int cap);
 
 #endif /* __PERF_CAP_H */
diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
index a562702b4841..94f9c8faedda 100644
--- a/tools/perf/util/symbol.c
+++ b/tools/perf/util/symbol.c
@@ -2452,8 +2452,7 @@ static bool symbol__read_kptr_restrict(void)
 {
 	bool value = false;
 	FILE *fp = fopen("/proc/sys/kernel/kptr_restrict", "r");
-	bool used_root;
-	bool cap_syslog = perf_cap__capable(CAP_SYSLOG, &used_root);
+	bool cap_syslog = perf_cap__capable(CAP_SYSLOG);
 
 	if (fp != NULL) {
 		char line[8];
diff --git a/tools/perf/util/util.c b/tools/perf/util/util.c
index 2c2a5c449ffd..8f7cd32f524d 100644
--- a/tools/perf/util/util.c
+++ b/tools/perf/util/util.c
@@ -378,15 +378,9 @@ int perf_event_paranoid(void)
 
 bool perf_event_paranoid_check(int max_level)
 {
-	bool used_root;
-
-	if (perf_cap__capable(CAP_SYS_ADMIN, &used_root))
-		return true;
-
-	if (!used_root && perf_cap__capable(CAP_PERFMON, &used_root))
-		return true;
-
-	return perf_event_paranoid() <= max_level;
+	return perf_cap__capable(CAP_SYS_ADMIN) ||
+	       perf_cap__capable(CAP_PERFMON) ||
+	       perf_event_paranoid() <= max_level;
 }
 
 int perf_tip(char **strp, const char *dirpath)
-- 
2.55.0.141.g00534a21ce-goog