perf dso: Guard against cache underflow on short reads in dso_cache__memcpy()

dso_cache__memcpy() computes cache_offset = offset - cache->offset,
then cache_size = min(cache->size - cache_offset, size).  The RB tree
lookup in __dso_cache__find() matches using the full
DSO__DATA_CACHE_SIZE window, but cache->size reflects the actual pread
return value from dso_cache__populate().

A short pread (e.g. near end-of-file) makes cache->size smaller than
DSO__DATA_CACHE_SIZE.  If a subsequent access targets an offset past
cache->offset + cache->size but within the DSO__DATA_CACHE_SIZE
window, the cache entry is found but cache_offset exceeds cache->size.
Since both are u64, the subtraction cache->size - cache_offset wraps
to a large value, min() selects the caller's size, and memcpy reads
out of bounds.

Return 0 for an offset past the valid cached data.  For a regular
file a short pread only happens at end-of-file, so 0 is what a direct
pread() at that offset would return: cached_io() stops its read loop
as on EOF.  Re-reading from the backing file would not help — a
second pread at the same offset returns the same short count.

Fixes: 366df72657 ("perf dso: Refactor dso_cache__read()")
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Reviewed-by: Ian Rogers <irogers@google.com>
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

tools/perf/util/dso.c

@@ -1014,7 +1014,20 @@ static ssize_t dso_cache__memcpy(struct dso_cache *cache, u64 offset, u8 *data,
 				 u64 size, bool out)
 {
 	u64 cache_offset = offset - cache->offset;
-	u64 cache_size   = min(cache->size - cache_offset, size);
+	u64 cache_size;
+
+	/*
+	 * The RB tree matches using DSO__DATA_CACHE_SIZE, but a short
+	 * pread may leave cache->size smaller.  For a regular file a
+	 * short pread only happens at end-of-file, so an offset past
+	 * the valid data is EOF: return 0, matching what a direct
+	 * pread() at that offset would return, and cached_io() then
+	 * stops its read loop.
+	 */
+	if (cache_offset >= cache->size)
+		return 0;
+
+	cache_size = min(cache->size - cache_offset, size);
 
 	if (out)
 		memcpy(data, cache->data + cache_offset, cache_size);