perf symbol-minimal: Fix double free in filename__read_build_id

Running the "perf script task-analyzer tests" with address sanitizer
showed a double free:
```
FAIL: "test_csv_extended_times" Error message: "Failed to find required string:'Out-Out;'."
=================================================================
==19190==ERROR: AddressSanitizer: attempting double-free on 0x50b000017b10 in thread T0:
    #0 0x55da9601c78a in free (perf+0x26078a) (BuildId: e7ef50e08970f017a96fde6101c5e2491acc674a)
    #1 0x55da96640c63 in filename__read_build_id tools/perf/util/symbol-minimal.c:221:2

0x50b000017b10 is located 0 bytes inside of 112-byte region [0x50b000017b10,0x50b000017b80)
freed by thread T0 here:
    #0 0x55da9601ce40 in realloc (perf+0x260e40) (BuildId: e7ef50e08970f017a96fde6101c5e2491acc674a)
    #1 0x55da96640ad6 in filename__read_build_id tools/perf/util/symbol-minimal.c:204:10

previously allocated by thread T0 here:
    #0 0x55da9601ca23 in malloc (perf+0x260a23) (BuildId: e7ef50e08970f017a96fde6101c5e2491acc674a)
    #1 0x55da966407e7 in filename__read_build_id tools/perf/util/symbol-minimal.c:181:9

SUMMARY: AddressSanitizer: double-free (perf+0x26078a) (BuildId: e7ef50e08970f017a96fde6101c5e2491acc674a) in free
==19190==ABORTING
FAIL: "invocation of perf script report task-analyzer --csv-summary csvsummary --summary-extended command failed" Error message: ""
FAIL: "test_csvsummary_extended" Error message: "Failed to find required string:'Out-Out;'."
---- end(-1) ----
132: perf script task-analyzer tests                                 : FAILED!
```

The buf_size if always set to phdr->p_filesz, but that may be 0
causing a free and realloc to return NULL. This is treated in
filename__read_build_id like a failure and the buffer is freed again.

To avoid this problem only grow buf, meaning the buf_size will never
be 0. This also reduces the number of memory (re)allocations.

Fixes: b691f64360 ("perf symbols: Implement poor man's ELF parser")
Signed-off-by: Ian Rogers <[email protected]>
Acked-by: Namhyung Kim <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

tools/perf/util/symbol-minimal.c

@@ -147,18 +147,19 @@ int filename__read_build_id(const char *filename, struct build_id *bid)
 			if (phdr->p_type != PT_NOTE)
 				continue;
 
-			buf_size = phdr->p_filesz;
 			offset = phdr->p_offset;
-			tmp = realloc(buf, buf_size);
-			if (tmp == NULL)
-				goto out_free;
-
-			buf = tmp;
+			if (phdr->p_filesz > buf_size) {
+				buf_size = phdr->p_filesz;
+				tmp = realloc(buf, buf_size);
+				if (tmp == NULL)
+					goto out_free;
+				buf = tmp;
+			}
 			fseek(fp, offset, SEEK_SET);
-			if (fread(buf, buf_size, 1, fp) != 1)
+			if (fread(buf, phdr->p_filesz, 1, fp) != 1)
 				goto out_free;
 
-			ret = read_build_id(buf, buf_size, bid, need_swap);
+			ret = read_build_id(buf, phdr->p_filesz, bid, need_swap);
 			if (ret == 0) {
 				ret = bid->size;
 				break;
@@ -199,18 +200,19 @@ int filename__read_build_id(const char *filename, struct build_id *bid)
 			if (phdr->p_type != PT_NOTE)
 				continue;
 
-			buf_size = phdr->p_filesz;
 			offset = phdr->p_offset;
-			tmp = realloc(buf, buf_size);
-			if (tmp == NULL)
-				goto out_free;
-
-			buf = tmp;
+			if (phdr->p_filesz > buf_size) {
+				buf_size = phdr->p_filesz;
+				tmp = realloc(buf, buf_size);
+				if (tmp == NULL)
+					goto out_free;
+				buf = tmp;
+			}
 			fseek(fp, offset, SEEK_SET);
-			if (fread(buf, buf_size, 1, fp) != 1)
+			if (fread(buf, phdr->p_filesz, 1, fp) != 1)
 				goto out_free;
 
-			ret = read_build_id(buf, buf_size, bid, need_swap);
+			ret = read_build_id(buf, phdr->p_filesz, bid, need_swap);
 			if (ret == 0) {
 				ret = bid->size;
 				break;