perf tools: Add helpers to use capabilities if present

Add utilities to help checking capabilities of the running procss. Make
perf link with libcap, if it is available. If no libcap-dev[el],
fallback to the geteuid() == 0 test used before.

Committer notes:

$ perf test python
18: 'import perf' in python : FAILED!
$ perf test -v python
Couldn't bump rlimit(MEMLOCK), failures may take place when creating BPF maps, etc
18: 'import perf' in python :
--- start ---
test child forked, pid 23288
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ImportError: /tmp/build/perf/python/perf.so: undefined symbol: cap_get_flag
test child finished with -1
---- end ----
'import perf' in python: FAILED!
$

This happens because differently from the perf binary generated with
this patch applied:

$ ldd /tmp/build/perf/perf | grep libcap
libcap.so.2 => /lib64/libcap.so.2 (0x00007f724a4ef000)
$

The python binding isn't linking with libcap:

$ ldd /tmp/build/perf/python/perf.so | grep libcap
$

So add 'cap' to the 'extra_libraries' variable in
tools/perf/util/setup.py, and rebuild:

$ perf test python
18: 'import perf' in python : Ok
$

If we explicitely disable libcap it also continues to work:

$ make NO_LIBCAP=1 -C tools/perf O=/tmp/build/perf install-bin
$ ldd /tmp/build/perf/perf | grep libcap
$ ldd /tmp/build/perf/python/perf.so | grep libcap
$ perf test python
18: 'import perf' in python : Ok
$

Signed-off-by: Igor Lubashev <[e-mail]>
Acked-by: Jiri Olsa <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Alexey Budankov <[e-mail]>
Cc: James Morris <[e-mail]>
Cc: Mathieu Poirier <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Suzuki Poulouse <[e-mail]>
Cc: [e-mail]
[ split from a larger patch ]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/Build

···
 perf-$(CONFIG_LZMA) += lzma.o
 perf-$(CONFIG_ZSTD) += zstd.o
 
+perf-$(CONFIG_LIBCAP) += cap.o
+
 perf-y += demangle-java.o
 perf-y += demangle-rust.o
 

tools/perf/util/cap.c

···
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * 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();
+
+	if (!caps)
+		return false;
+
+	if (cap_get_flag(caps, cap, CAP_EFFECTIVE, &val) != 0)
+		val = CAP_CLEAR;
+
+	if (cap_free(caps) != 0)
+		return false;
+
+	return val == CAP_SET;
+}
+
+#endif  /* HAVE_LIBCAP_SUPPORT */

tools/perf/util/cap.h

···
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __PERF_CAP_H
+#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 */
+
+#endif /* __PERF_CAP_H */

tools/perf/util/event.h

···
 void event_attr_init(struct perf_event_attr *attr);
 
 int perf_event_paranoid(void);
+bool perf_event_paranoid_check(int max_level);
 
 extern int sysctl_perf_event_max_stack;
 extern int sysctl_perf_event_max_contexts_per_stack;

tools/perf/util/python-ext-sources

···
 
 util/python.c
 ../lib/ctype.c
+util/cap.c
 util/evlist.c
 util/evsel.c
 util/cpumap.c

tools/perf/util/setup.py

···
 extra_libraries = []
 if '-DHAVE_LIBNUMA_SUPPORT' in cflags:
     extra_libraries = [ 'numa' ]
+if '-DHAVE_LIBCAP_SUPPORT' in cflags:
+    extra_libraries += [ 'cap' ]
 
 perf = Extension('perf',
 		  sources = ext_sources,

tools/perf/util/util.c

···
 #include <string.h>
 #include <errno.h>
 #include <limits.h>
+#include <linux/capability.h>
 #include <linux/kernel.h>
 #include <linux/log2.h>
 #include <linux/time64.h>
 #include <unistd.h>
+#include "cap.h"
 #include "strlist.h"
 #include "string2.h"
 
···
 
 	return value;
 }
+
+bool perf_event_paranoid_check(int max_level)
+{
+	return perf_cap__capable(CAP_SYS_ADMIN) ||
+			perf_event_paranoid() <= max_level;
+}
+
 static int
 fetch_ubuntu_kernel_version(unsigned int *puint)
 {