linux-kernel - [PATCH] perf bpf augmented_raw_syscalls: Add an assert to make sure sizeof(saddr) is a power of two.

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Date:   Mon, 21 Aug 2023 10:48:06 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Ian Rogers <irogers@...gle.com>
Cc:     Adrian Hunter <adrian.hunter@...el.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [PATCH] perf bpf augmented_raw_syscalls: Add an assert to make sure
 sizeof(saddr) is a power of two.

We're using the BPF verifier suggestion:

    22: (85) call bpf_probe_read#4
    R2 min value is negative, either use unsigned or 'var &= const'

That works only when const is a (power of two - 1) so add an assert to
make sure that that is the case.

Suggested-by: Ian Rogers <irogers@...gle.com>
Cc: Adrian Hunter <adrian.hunter@...el.com>
Cc: Jiri Olsa <jolsa@...nel.org>
Cc: Namhyung Kim <namhyung@...nel.org>
Link: https://lore.kernel.org/lkml/
Signed-off-by: Arnaldo Carvalho de Melo <acme@...hat.com>
---
 tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c b/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c
index 9c1d0b271b20f693..43549b63b433d81e 100644
--- a/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c
+++ b/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c
@@ -10,6 +10,16 @@
 #include <bpf/bpf_helpers.h>
 #include <linux/limits.h>
 
+/**
+ * is_power_of_2() - check if a value is a power of two
+ * @n: the value to check
+ *
+ * Determine whether some value is a power of two, where zero is *not*
+ * considered a power of two.  Return: true if @n is a power of 2, otherwise
+ * false.
+ */
+#define is_power_of_2(n) (n != 0 && ((n & (n - 1)) == 0))
+
 #define MAX_CPUS  4096
 
 // FIXME: These should come from system headers
@@ -187,6 +197,7 @@ int sys_enter_connect(struct syscall_enter_args *args)
         if (augmented_args == NULL)
                 return 1; /* Failure: don't filter */
 
+	_Static_assert(is_power_of_2(sizeof(augmented_args->saddr)), "sizeof(augmented_args->saddr) needs to be a power of two");
 	socklen &= sizeof(augmented_args->saddr) - 1;
 
 	bpf_probe_read(&augmented_args->saddr, socklen, sockaddr_arg);
-- 
2.37.1