It has been observed [1] that the recent dwarves changes
that skip BTF encoding for functions that have optimized-out
parameters are too aggressive, leading to missing kfuncs
which generate warnings and a BPF selftest failure.
Here a different approach is used; we observe that
just because a function does not _use_ a parameter,
it does not mean it was not passed to it. What we
are really keen to detect are cases where the calling
conventions are violated such that a function will
not have parameter values in the expected registers.
In such cases, tracing and kfunc behaviour will be
unstable. We are not worried about parameters being
optimized out, provided that optimization does not
lead to other parameters being passed via
unexpected registers.
So this series attempts to detect such cases by
examining register (DW_OP_regX) values for
parameters where available; if these match
expectations, the function is deemed safe to add to
BTF, even if parameters are optimized out.
Using this approach, the only functions that
BTF generation is suppressed for are
1. those with parameters that violate calling
conventions where present; and
2. those which have multiple inconsistent prototypes.
With these changes, running pahole on a gcc-built
vmlinux skips
- 1164 functions due to multiple inconsistent function
prototypes. Most of these are "."-suffixed optimized
fuctions.
- 331 functions due to unexpected register usage
For a clang-built kernel, the numbers are
- 539 functions with inconsistent prototypes are skipped
- 209 functions with unexpected register usage are skipped
One complication is that functions that are passed
structs (or typedef structs) can use multiple registers
to pass those structures. Examples include
bpf_lsm_socket_getpeersec_stream() (passing a typedef
struct sockptr_t) and the bpf_testmod_test_struct_arg_1
function in bpf_testmod. Because multiple registers
are used to represent the structure, this throws
off expectations for any subsequent parameter->register
mappings. To handle this, simply exempt functions
that have struct (or typedef struct) parameters from
our register checks.
Note to test this series on bpf-next, the following
commit should be reverted (reverting the revert
so that the flags are added to BTF encoding when
using pahole v1.25):
commit 1f5dfcc78ab4 ("Revert "bpf: Add --skip_encoding_btf_inconsistent_proto, --btf_gen_optimized to pahole flags for v1.25"")
With these changes we also see tracing_struct now pass:
$ sudo ./test_progs -t tracing_struct
#233 tracing_struct:OK
Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED
Further testing is needed - along with support for additional
parameter index -> DWARF reg for more platforms.
Future work could also add annotations for optimized-out
parameters via BTF tags to help guide tracing.
[1] https://lore.kernel.org/bpf/CAADnVQ+hfQ9LEmEFXneB7hm17NvRniXSShrHLaM-1BrguLjLQw@xxxxxxxxxxxxxx/
Alan Maguire (4):
dwarf_loader: mark functions that do not use expected registers for
params
btf_encoder: exclude functions with unexpected param register use not
optimizations
pahole: update descriptions for btf_gen_optimized,
skip_encoding_btf_inconsistent_proto
pahole: update man page for options also
btf_encoder.c | 24 +++++++---
dwarf_loader.c | 109 ++++++++++++++++++++++++++++++++++++++++++---
dwarves.h | 5 +++
man-pages/pahole.1 | 4 +-
pahole.c | 4 +-
5 files changed, 129 insertions(+), 17 deletions(-)
--
2.31.1
