Re: [PATCH v1] perf cap: Tidy up and improve capability testing
From: Ian Rogers
Date: Mon Jul 29 2024 - 14:16:42 EST
On Mon, Jul 29, 2024 at 11:14 AM Ian Rogers <irogers@xxxxxxxxxx> 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, to reduce calls or refactor similar to:
> https://lore.kernel.org/lkml/20240729004127.238611-3-namhyung@xxxxxxxxxx/
>
> 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..1ef8af0ccde9 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 265
> +#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) {
Should be >=, will fix in v2.
Thanks,
Ian
> + 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
>