Commit e25ebda7 authored by Ian Rogers's avatar Ian Rogers Committed by Arnaldo Carvalho de Melo
Browse files

perf cap: Tidy up and improve capability testing 



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.

Signed-off-by: default avatarIan Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Cc: Changbin Du <changbin.du@huawei.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Leo Yan <leo.yan@linux.dev>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Oliver Upton <oliver.upton@linux.dev>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240806220614.831914-1-irogers@google.com


Signed-off-by: default avatarArnaldo Carvalho de Melo <acme@redhat.com>
parent 8b1042c4

tools/perf/Makefile.config

+0 −11
Original line number Diff line number Diff line
@@ -1031,17 +1031,6 @@ 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

tools/perf/builtin-ftrace.c

+14 −14
Original line number Diff line number Diff line
@@ -63,20 +63,21 @@ static void ftrace__workload_exec_failed_signal(int signo __maybe_unused, 
	done = true;

}



static int check_ftrace_capable(void)

static bool check_ftrace_capable(void)

{

	if (!(perf_cap__capable(CAP_PERFMON) ||

	      perf_cap__capable(CAP_SYS_ADMIN))) {

	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",

#ifdef HAVE_LIBCAP_SUPPORT

		"users with the CAP_PERFMON or CAP_SYS_ADMIN capability"

#else

		"root"

#endif

		used_root ? "root"

			  : "users with the CAP_PERFMON or CAP_SYS_ADMIN capability"

		);

		return -1;

	}

	return 0;

	return false;

}



static int __write_tracing_file(const char *name, const char *val, bool append)
@@ -1579,8 +1580,7 @@ int cmd_ftrace(int argc, const char **argv) 
	signal(SIGCHLD, sig_handler);

	signal(SIGPIPE, sig_handler);



	ret = check_ftrace_capable();

	if (ret < 0)

	if (!check_ftrace_capable())

		return -1;



	ret = perf_config(perf_ftrace_config, &ftrace);

tools/perf/util/Build

+1 −1
Original line number Diff line number Diff line
@@ -223,7 +223,7 @@ 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

tools/perf/util/cap.c

+44 −19
Original line number Diff line number Diff line
@@ -3,27 +3,52 @@ 
 * Capability utilities

 */



#ifdef HAVE_LIBCAP_SUPPORT



#include "cap.h"

#include <stdbool.h>

#include <sys/capability.h>

#include "debug.h"

#include <errno.h>

#include <string.h>

#include <unistd.h>

#include <linux/capability.h>

#include <sys/syscall.h>



bool perf_cap__capable(cap_value_t cap)

{

	cap_flag_value_t val;

	cap_t caps = cap_get_proc();

#ifndef SYS_capget

#define SYS_capget 90

#endif



	if (!caps)

		return false;

#define MAX_LINUX_CAPABILITY_U32S _LINUX_CAPABILITY_U32S_3



	if (cap_get_flag(caps, cap, CAP_EFFECTIVE, &val) != 0)

		val = CAP_CLEAR;

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;

	}



	if (cap_free(caps) != 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;



	return val == CAP_SET;

		}



#endif  /* HAVE_LIBCAP_SUPPORT */
 
	} else {

		cap_val = data[0].effective;

	}

	return (cap_val & (1 << (cap & 0x1f))) != 0;

}

tools/perf/util/cap.h

+3 −20
Original line number Diff line number Diff line
@@ -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 */