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libbpf: Implement bpf_usdt_arg_size BPF function

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Information about USDT argument size is implicitly stored in
__bpf_usdt_arg_spec, but currently it's not accessbile to BPF programs
that use USDT.

Implement bpf_sdt_arg_size() that returns the size of an USDT argument
in bytes.

v1->v2:
  * do not add __bpf_usdt_arg_spec() helper

v1: https://lore.kernel.org/bpf/20250220215904.3362709-1-ihor.solodrai@linux.dev/

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20250224235756.2612606-1-ihor.solodrai@linux.dev
This commit is contained in:
Ihor Solodrai 2025-02-24 15:57:55 -08:00 committed by Andrii Nakryiko
parent 4580f4e0eb
commit b62dff1440
1 changed files with 32 additions and 0 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

32
tools/lib/bpf/usdt.bpf.h
View file

@ -108,6 +108,38 @@ int bpf_usdt_arg_cnt(struct pt_regs *ctx)
return spec->arg_cnt;
}
/* Returns the size in bytes of the #*arg_num* (zero-indexed) USDT argument.
* Returns negative error if argument is not found or arg_num is invalid.
*/
static __always_inline
int bpf_usdt_arg_size(struct pt_regs *ctx, __u64 arg_num)
{
struct __bpf_usdt_arg_spec *arg_spec;
struct __bpf_usdt_spec *spec;
int spec_id;
spec_id = __bpf_usdt_spec_id(ctx);
if (spec_id < 0)
return -ESRCH;
spec = bpf_map_lookup_elem(&__bpf_usdt_specs, &spec_id);
if (!spec)
return -ESRCH;
if (arg_num >= BPF_USDT_MAX_ARG_CNT)
return -ENOENT;
barrier_var(arg_num);
if (arg_num >= spec->arg_cnt)
return -ENOENT;
arg_spec = &spec->args[arg_num];
/* arg_spec->arg_bitshift = 64 - arg_sz * 8
* so: arg_sz = (64 - arg_spec->arg_bitshift) / 8
*/
return (unsigned int)(64 - arg_spec->arg_bitshift) / 8;
}
/* Fetch USDT argument #*arg_num* (zero-indexed) and put its value into *res.
* Returns 0 on success; negative error, otherwise.
* On error *res is guaranteed to be set to zero.