[PATCH 03/15] perf symbols: Use fixed buffer in sysfs__read_build_id() for no-libelf build

[Arnaldo Carvalho de Melo]

From: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>

sysfs__read_build_id() in the no-libelf build path uses fstat() to get
the file size and then allocates an exact-sized buffer.  Sysfs pseudo-
files (/sys/kernel/notes) often report st_size=0 or a page-sized value
(4096) regardless of actual content.  When st_size is 0, malloc(0)
returns a zero-sized allocation and read(fd, buf, 0) returns 0, which
matches the expected (ssize_t)buf_size check, then read_build_id() is
called with a zero-length buffer and never finds any notes.

Replace the fstat + malloc approach with a fixed BUFSIZ stack buffer and
a single read(), matching the strategy used by the libelf-based
sysfs__read_build_id() in symbol-elf.c.  This also eliminates the
malloc/free overhead for what is always a small sysfs file.

Reported-by: sashiko-bot <sashiko-bot@xxxxxxxxxx>
Fixes: b691f64360ecec49 ("perf symbols: Implement poor man's ELF parser")
Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
Assisted-by: Claude Opus 4.6 <noreply@xxxxxxxxxxxxx>
Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
---
 tools/perf/util/symbol-minimal.c | 41 +++++++++++++++++++-------------
 1 file changed, 24 insertions(+), 17 deletions(-)

diff --git a/tools/perf/util/symbol-minimal.c b/tools/perf/util/symbol-minimal.c
index 0a71d146395271a6..ea2de3d50d33cf33 100644
--- a/tools/perf/util/symbol-minimal.c
+++ b/tools/perf/util/symbol-minimal.c
@@ -220,29 +220,36 @@ int sysfs__read_build_id(const char *filename, struct build_id *bid)
 {
 	int fd;
 	int ret = -1;
-	struct stat stbuf;
-	size_t buf_size;
-	void *buf;
+	/*
+	 * read_build_id() casts this buffer to a u32 note header pointer,
+	 * so it must be aligned for u32 access.
+	 *
+	 * Accessing a char[] through a u32* is technically type-punning
+	 * under C strict aliasing rules, but perf unconditionally builds
+	 * with -fno-strict-aliasing (Makefile.config), so this is safe.
+	 *
+	 * This file is only compiled when libelf is not available — the
+	 * common case uses the libelf-based path in symbol-elf.c instead.
+	 */
+	char buf[BUFSIZ] __aligned(4);
+	ssize_t len;
 
 	fd = open(filename, O_RDONLY);
 	if (fd < 0)
 		return -1;
 
-	if (fstat(fd, &stbuf) < 0)
-		goto out;
-
-	buf_size = stbuf.st_size;
-	buf = malloc(buf_size);
-	if (buf == NULL)
-		goto out;
-
-	if (read(fd, buf, buf_size) != (ssize_t) buf_size)
-		goto out_free;
+	/*
+	 * Use a fixed buffer and a single read() instead of fstat() + malloc(),
+	 * because sysfs pseudo-files often report st_size=0 or 4096
+	 * regardless of actual content size.
+	 *
+	 * BUFSIZ (8192) is more than sufficient: a sysfs build-id note is
+	 * ~36 bytes (12-byte note header + 4-byte "GNU" name + 20-byte SHA1).
+	 */
+	len = read(fd, buf, sizeof(buf));
+	if (len > 0)
+		ret = read_build_id(buf, len, bid, false);
 
-	ret = read_build_id(buf, buf_size, bid, false);
-out_free:
-	free(buf);
-out:
 	close(fd);
 	return ret;
 }
-- 
2.54.0