Re: [PATCH v5 0/3] Use BTF to trim return values

On Tue,  1 Sep 2026 21:46:01 +0800
Donglin Peng [off-list ref] wrote:

From: pengdonglin <redacted>

The funcgraph-retval option currently records one machine-word return
value without considering the function's declared return type. As a
result, void-returning functions can produce meaningless output, and
return values narrower than a general-purpose register can include
undefined high bits.

This series uses kernel BTF to identify the return type of traced
functions and to improve funcgraph-retval output:

  - Do not print a return value for functions with a void return type.
  - Trim integer, enum, struct, and union values to the available return
    value width when the BTF type provides suitable size information.
  - Format integer, enum, and boolean values according to their BTF
    encoding.
  - Mark values as "(trunc)" when the return type is wider than the
    value captured by the function graph tracer.

Hmm, I really don't like the "(trunc)" If it's the real return type, then
it should just print what the real size is. No need to state it was truncated.

-- Steve

Here is an output comparison:

Before:
 # perf ftrace -G vfs_read --graph-opts retval
 ...
 1)               |   touch_atime() {
 1)               |     atime_needs_update() {
 1)   0.069 us    |       make_vfsuid(); /* ret=0x0 */
 1)   0.067 us    |       make_vfsgid(); /* ret=0x0 */
 1)               |       current_time() {
 1)   0.197 us    |         ktime_get_coarse_real_ts64_mg(); /* ret=0x187f886aec3ed6f5 */
 1)   0.352 us    |       } /* current_time ret=0x69380753 */
 1)   0.792 us    |     } /* atime_needs_update ret=0x0 */
 1)   0.937 us    |   } /* touch_atime ret=0x0 */

After:
 # perf ftrace -G vfs_read --graph-opts retval
 ...
 2)               |   touch_atime() {
 2)               |     atime_needs_update() {
 2)   0.070 us    |       make_vfsuid(); /* ret=0x0 */
 2)   0.070 us    |       make_vfsgid(); /* ret=0x0 */
 2)               |       current_time() {
 2)   0.162 us    |         ktime_get_coarse_real_ts64_mg();
 2)   0.312 us    |       } /* current_time ret=0x69380649(trunc) */
 2)   0.753 us    |     } /* atime_needs_update ret=false */
 2)   0.899 us    |   } /* touch_atime */