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Message-ID: <CAP-5=fUWdyp++BKgwZoKWv_mYNGhKj2LKG8p2Ghfg74T+6PLXg@mail.gmail.com>
Date: Tue, 20 Aug 2024 09:23:51 -0700
From: Ian Rogers <irogers@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>,
Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>,
Kan Liang <kan.liang@...ux.intel.com>, James Clark <james.clark@....com>,
Oliver Upton <oliver.upton@...ux.dev>, Leo Yan <leo.yan@...ux.dev>,
Changbin Du <changbin.du@...wei.com>, Athira Rajeev <atrajeev@...ux.vnet.ibm.com>,
linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] perf cap: Tidy up and improve capability testing
On Tue, Aug 6, 2024 at 3:06 PM Ian Rogers <irogers@...gle.com> wrote:
>
> 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: Ian Rogers <irogers@...gle.com>
Ping.
Thanks,
Ian
> ---
> v3: rebase resolving builtin-ftrace.c conflicts and incorporating
> change in:
> https://lore.kernel.org/lkml/20240729004127.238611-3-namhyung@kernel.org/
> v2: fix syscall number and '>' should have been '>='.
> ---
> tools/perf/Makefile.config | 11 -------
> tools/perf/builtin-ftrace.c | 28 ++++++++---------
> 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, 75 insertions(+), 72 deletions(-)
>
> diff --git a/tools/perf/Makefile.config b/tools/perf/Makefile.config
> index fa679db61f62..4eb1fc897baf 100644
> --- a/tools/perf/Makefile.config
> +++ b/tools/perf/Makefile.config
> @@ -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
> diff --git a/tools/perf/builtin-ftrace.c b/tools/perf/builtin-ftrace.c
> index a615c405d98f..88a87bf387d2 100644
> --- a/tools/perf/builtin-ftrace.c
> +++ b/tools/perf/builtin-ftrace.c
> @@ -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))) {
> - 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
> + 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;
> - }
> - 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);
> diff --git a/tools/perf/util/Build b/tools/perf/util/Build
> index b24360c04aae..adbaf7748019 100644
> --- a/tools/perf/util/Build
> +++ b/tools/perf/util/Build
> @@ -221,7 +221,7 @@ 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.rc2.264.g509ed76dc8-goog
>
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