[PATCH v6 0/4] Use BTF to trim return values

[Donglin Peng <dolinux.peng@gmail.com>]

From: pengdonglin <pengdonglin@xiaomi.com>

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.

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 */

Note that funcgraph-retval still captures only the return value exposed
by ftrace_regs_get_return_value(). Return values that require multiple
registers are therefore still incomplete and are marked "(trunc)" when
the BTF type indicates that the declared type is wider than one machine
word.

Changelog:

v6:
  - Avoid BTF lookup in atomic context or with interrupts disabled
  - Add the atomic-context fix as a separate patch.

v5:
  - Link: https://lore.kernel.org/linux-trace-kernel/20260901134604.4052265-1-dolinux.peng@gmail.com/
  - Add an inline fallback for btf_trim_retval() when
    CONFIG_DEBUG_INFO_BTF is disabled.
  - Release the BTF reference after return type lookup.

v4:
  - Link: https://lore.kernel.org/all/20251215034153.2367756-1-dolinux.peng@gmail.com/
  - Build trace_btf.c when CONFIG_DEBUG_INFO_BTF is enabled.
  - Remove the redundant BPF_SYSCALL dependency from
    CONFIG_PROBE_EVENTS_BTF_ARGS.
  - Update the funcgraph-retval documentation and cover letter.

v3:
  - Link: https://lore.kernel.org/all/20251209121349.525641-1-dolinux.peng@gmail.com/
  - Print the return value based on its type for human readability,
    thanks Masami.
  - Update the documentation and cover letter.

v2:
  - Link: https://lore.kernel.org/all/20251208131917.2444620-1-dolinux.peng@gmail.com/
  - Update the funcgraph-retval documentation.
  - Revise the cover letter.

v1:
  - Link: https://lore.kernel.org/all/20251207142742.229924-1-dolinux.peng@gmail.com/

pengdonglin (4):
  tracing: Avoid BTF lookup in atomic context
  ftrace: Build trace_btf.c when CONFIG_DEBUG_INFO_BTF is enabled
  fgraph: Enhance funcgraph-retval with BTF-based type-aware output
  tracing: Update funcgraph-retval documentation

 Documentation/trace/ftrace.rst       | 96 +++++++++++++++++-----------
 kernel/trace/Kconfig                 |  2 +-
 kernel/trace/Makefile                |  2 +-
 kernel/trace/trace_btf.c             | 86 +++++++++++++++++++++++++
 kernel/trace/trace_btf.h             | 18 ++++++
 kernel/trace/trace_functions_graph.c | 56 ++++++++++++----
 6 files changed, 207 insertions(+), 53 deletions(-)

-- 
2.34.1