[PATCH v2] perf cap: Tidy up and improve capability testing
From: Ian Rogers
Date: Mon Jul 29 2024 - 14:19:45 EST
Remove dependence on libcap. libcap is only used to query whether a
capability is supported, which is just 1 capget system call.
If the capget system call fails, fall back on root permission
checking. Previously if libcap fails then the permission is assumed
not present which may be pessimistic/wrong.
Add a used_root out argument to perf_cap__capable to say whether the
fall back root check was used. This allows the correct error message,
"root" vs "users with the CAP_PERFMON or CAP_SYS_ADMIN capability", to
be selected.
Tidy uses of perf_cap__capable so that tests aren't repeated if capget
isn't supported, to reduce calls or refactor similar to:
https://lore.kernel.org/lkml/20240729004127.238611-3-namhyung@xxxxxxxxxx/
---
v2: fix syscall number and '>' should have been '>='
Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
---
tools/perf/Makefile.config | 11 -------
tools/perf/builtin-ftrace.c | 44 ++++++++++++--------------
tools/perf/util/Build | 2 +-
tools/perf/util/cap.c | 63 ++++++++++++++++++++++++++-----------
tools/perf/util/cap.h | 23 ++------------
tools/perf/util/symbol.c | 8 ++---
tools/perf/util/util.c | 12 +++++--
7 files changed, 81 insertions(+), 82 deletions(-)
diff --git a/tools/perf/Makefile.config b/tools/perf/Makefile.config
index a4829b6532d8..a9517272f80c 100644
--- a/tools/perf/Makefile.config
+++ b/tools/perf/Makefile.config
@@ -1018,17 +1018,6 @@ ifndef NO_LIBZSTD
endif
endif
-ifndef NO_LIBCAP
- ifeq ($(feature-libcap), 1)
- CFLAGS += -DHAVE_LIBCAP_SUPPORT
- EXTLIBS += -lcap
- $(call detected,CONFIG_LIBCAP)
- else
- $(warning No libcap found, disables capability support, please install libcap-devel/libcap-dev)
- NO_LIBCAP := 1
- endif
-endif
-
ifndef NO_BACKTRACE
ifeq ($(feature-backtrace), 1)
CFLAGS += -DHAVE_BACKTRACE_SUPPORT
diff --git a/tools/perf/builtin-ftrace.c b/tools/perf/builtin-ftrace.c
index eb30c8eca488..435208288d24 100644
--- a/tools/perf/builtin-ftrace.c
+++ b/tools/perf/builtin-ftrace.c
@@ -560,6 +560,23 @@ static void select_tracer(struct perf_ftrace *ftrace)
pr_debug("%s tracer is used\n", ftrace->tracer);
}
+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))
+ return true;
+
+ pr_err("ftrace only works for %s!\n",
+ used_root ? "root"
+ : "users with the CAP_PERFMON or CAP_SYS_ADMIN capability"
+ );
+ return -1;
+}
+
static int __cmd_ftrace(struct perf_ftrace *ftrace)
{
char *trace_file;
@@ -569,18 +586,6 @@ static int __cmd_ftrace(struct perf_ftrace *ftrace)
.events = POLLIN,
};
- if (!(perf_cap__capable(CAP_PERFMON) ||
- perf_cap__capable(CAP_SYS_ADMIN))) {
- pr_err("ftrace only works for %s!\n",
-#ifdef HAVE_LIBCAP_SUPPORT
- "users with the CAP_PERFMON or CAP_SYS_ADMIN capability"
-#else
- "root"
-#endif
- );
- return -1;
- }
-
select_tracer(ftrace);
if (reset_tracing_files(ftrace) < 0) {
@@ -885,18 +890,6 @@ static int __cmd_latency(struct perf_ftrace *ftrace)
};
int buckets[NUM_BUCKET] = { };
- if (!(perf_cap__capable(CAP_PERFMON) ||
- perf_cap__capable(CAP_SYS_ADMIN))) {
- pr_err("ftrace only works for %s!\n",
-#ifdef HAVE_LIBCAP_SUPPORT
- "users with the CAP_PERFMON or CAP_SYS_ADMIN capability"
-#else
- "root"
-#endif
- );
- return -1;
- }
-
trace_fd = prepare_func_latency(ftrace);
if (trace_fd < 0)
goto out;
@@ -1197,6 +1190,9 @@ int cmd_ftrace(int argc, const char **argv)
INIT_LIST_HEAD(&ftrace.graph_funcs);
INIT_LIST_HEAD(&ftrace.nograph_funcs);
+ if (!check_ftrace_capable())
+ return -1;
+
signal(SIGINT, sig_handler);
signal(SIGUSR1, sig_handler);
signal(SIGCHLD, sig_handler);
diff --git a/tools/perf/util/Build b/tools/perf/util/Build
index 0f18fe81ef0b..91ce0ab4defc 100644
--- a/tools/perf/util/Build
+++ b/tools/perf/util/Build
@@ -220,7 +220,7 @@ perf-util-$(CONFIG_ZLIB) += zlib.o
perf-util-$(CONFIG_LZMA) += lzma.o
perf-util-$(CONFIG_ZSTD) += zstd.o
-perf-util-$(CONFIG_LIBCAP) += cap.o
+perf-util-y += cap.o
perf-util-$(CONFIG_CXX_DEMANGLE) += demangle-cxx.o
perf-util-y += demangle-ocaml.o
diff --git a/tools/perf/util/cap.c b/tools/perf/util/cap.c
index c3ba841bbf37..7574a67651bc 100644
--- a/tools/perf/util/cap.c
+++ b/tools/perf/util/cap.c
@@ -3,27 +3,52 @@
* Capability utilities
*/
-#ifdef HAVE_LIBCAP_SUPPORT
-
#include "cap.h"
-#include <stdbool.h>
-#include <sys/capability.h>
-
-bool perf_cap__capable(cap_value_t cap)
-{
- cap_flag_value_t val;
- cap_t caps = cap_get_proc();
+#include "debug.h"
+#include <errno.h>
+#include <string.h>
+#include <unistd.h>
+#include <linux/capability.h>
+#include <sys/syscall.h>
- if (!caps)
- return false;
+#ifndef SYS_capget
+#define SYS_capget 90
+#endif
- if (cap_get_flag(caps, cap, CAP_EFFECTIVE, &val) != 0)
- val = CAP_CLEAR;
+#define MAX_LINUX_CAPABILITY_U32S _LINUX_CAPABILITY_U32S_3
- if (cap_free(caps) != 0)
- return false;
-
- return val == CAP_SET;
+bool perf_cap__capable(int cap, bool *used_root)
+{
+ struct __user_cap_header_struct header = {
+ .version = _LINUX_CAPABILITY_VERSION_3,
+ .pid = getpid(),
+ };
+ 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 &&
+ header.version == _LINUX_CAPABILITY_VERSION_1)
+ continue;
+
+ pr_debug2("capget syscall failed (%s - %d) fall back on root check\n",
+ strerror(errno), errno);
+ *used_root = true;
+ return geteuid() == 0;
+ }
+
+ /* Extract the relevant capability bit. */
+ if (cap >= 32) {
+ if (header.version == _LINUX_CAPABILITY_VERSION_3) {
+ cap_val = data[1].effective;
+ } else {
+ /* Capability beyond 32 is requested but only 32 are supported. */
+ return false;
+ }
+ } else {
+ cap_val = data[0].effective;
+ }
+ return (cap_val & (1 << (cap & 0x1f))) != 0;
}
-
-#endif /* HAVE_LIBCAP_SUPPORT */
diff --git a/tools/perf/util/cap.h b/tools/perf/util/cap.h
index ae52878c0b2e..0c6a1ff55f07 100644
--- a/tools/perf/util/cap.h
+++ b/tools/perf/util/cap.h
@@ -3,26 +3,6 @@
#define __PERF_CAP_H
#include <stdbool.h>
-#include <linux/capability.h>
-#include <linux/compiler.h>
-
-#ifdef HAVE_LIBCAP_SUPPORT
-
-#include <sys/capability.h>
-
-bool perf_cap__capable(cap_value_t cap);
-
-#else
-
-#include <unistd.h>
-#include <sys/types.h>
-
-static inline bool perf_cap__capable(int cap __maybe_unused)
-{
- return geteuid() == 0;
-}
-
-#endif /* HAVE_LIBCAP_SUPPORT */
/* For older systems */
#ifndef CAP_SYSLOG
@@ -33,4 +13,7 @@ static inline bool perf_cap__capable(int cap __maybe_unused)
#define CAP_PERFMON 38
#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);
+
#endif /* __PERF_CAP_H */
diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
index 19eb623e0826..a18927d792af 100644
--- a/tools/perf/util/symbol.c
+++ b/tools/perf/util/symbol.c
@@ -2425,14 +2425,14 @@ 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);
if (fp != NULL) {
char line[8];
if (fgets(line, sizeof(line), fp) != NULL)
- value = perf_cap__capable(CAP_SYSLOG) ?
- (atoi(line) >= 2) :
- (atoi(line) != 0);
+ value = cap_syslog ? (atoi(line) >= 2) : (atoi(line) != 0);
fclose(fp);
}
@@ -2440,7 +2440,7 @@ static bool symbol__read_kptr_restrict(void)
/* Per kernel/kallsyms.c:
* we also restrict when perf_event_paranoid > 1 w/o CAP_SYSLOG
*/
- if (perf_event_paranoid() > 1 && !perf_cap__capable(CAP_SYSLOG))
+ if (perf_event_paranoid() > 1 && !cap_syslog)
value = true;
return value;
diff --git a/tools/perf/util/util.c b/tools/perf/util/util.c
index 4f561e5e4162..9d55a13787ce 100644
--- a/tools/perf/util/util.c
+++ b/tools/perf/util/util.c
@@ -325,9 +325,15 @@ int perf_event_paranoid(void)
bool perf_event_paranoid_check(int max_level)
{
- return perf_cap__capable(CAP_SYS_ADMIN) ||
- perf_cap__capable(CAP_PERFMON) ||
- perf_event_paranoid() <= 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;
}
static int
--
2.46.0.rc1.232.g9752f9e123-goog