linux-kernel - [PATCH] perf bpf: Remove undefined behavior from bpf_perf_object_

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Date:   Tue, 26 Jul 2022 15:09:21 -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>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, Andrii Nakryiko <andrii@...nel.org>,
        Christy Lee <christylee@...com>,
        Alexei Starovoitov <ast@...nel.org>,
        Miaoqian Lin <linmq006@...il.com>,
        Ian Rogers <irogers@...gle.com>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org, llvm@...ts.linux.dev
Cc:     Stephane Eranian <eranian@...gle.com>
Subject: [PATCH] perf bpf: Remove undefined behavior from bpf_perf_object__next

bpf_perf_object__next folded the last element in the list test with the
empty list test. However, this meant that offsets were computed against
null and that a struct list_head was compared against a struct
bpf_perf_object. Working around this with clang's undefined behavior
sanitizer required -fno-sanitize=null and -fno-sanitize=object-size.

Remove the undefined behavior by using the regular Linux list APIs and
handling the starting case separately from the end testing case. Looking
at uses like bpf_perf_object__for_each, as the constant NULL or non-NULL
argument can be constant propagated the code is no less efficient.

Signed-off-by: Ian Rogers <irogers@...gle.com>
---
 tools/perf/util/bpf-loader.c | 18 +++++++-----------
 1 file changed, 7 insertions(+), 11 deletions(-)

diff --git a/tools/perf/util/bpf-loader.c b/tools/perf/util/bpf-loader.c
index f8ad581ea247..cdd6463a5b68 100644
--- a/tools/perf/util/bpf-loader.c
+++ b/tools/perf/util/bpf-loader.c
@@ -63,20 +63,16 @@ static struct hashmap *bpf_map_hash;
 static struct bpf_perf_object *
 bpf_perf_object__next(struct bpf_perf_object *prev)
 {
-	struct bpf_perf_object *next;
-
-	if (!prev)
-		next = list_first_entry(&bpf_objects_list,
-					struct bpf_perf_object,
-					list);
-	else
-		next = list_next_entry(prev, list);
+	if (!prev) {
+		if (list_empty(&bpf_objects_list))
+			return NULL;
 
-	/* Empty list is noticed here so don't need checking on entry. */
-	if (&next->list == &bpf_objects_list)
+		return list_first_entry(&bpf_objects_list, struct bpf_perf_object, list);
+	}
+	if (list_is_last(&prev->list, &bpf_objects_list))
 		return NULL;
 
-	return next;
+	return list_next_entry(prev, list);
 }
 
 #define bpf_perf_object__for_each(perf_obj, tmp)	\
-- 
2.37.1.359.gd136c6c3e2-goog