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libbpf: Simplify BPF_CORE_READ_BITFIELD_PROBED usage

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Streamline BPF_CORE_READ_BITFIELD_PROBED interface to follow
BPF_CORE_READ_BITFIELD (direct) and BPF_CORE_READ, in general, i.e., just
return read result or 0, if underlying bpf_probe_read() failed.

In practice, real applications rarely check bpf_probe_read() result, because
it has to always work or otherwise it's a bug. So propagating internal
bpf_probe_read() error from this macro hurts usability without providing real
benefits in practice. This patch fixes the issue and simplifies usage,
noticeable even in selftest itself.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20191106201500.2582438-1-andriin@fb.com
This commit is contained in:
Andrii Nakryiko 2019-11-06 12:15:00 -08:00 committed by Alexei Starovoitov
parent 65a052d537
commit ed57802121
2 changed files with 18 additions and 28 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

27
tools/lib/bpf/bpf_core_read.h
View file

@ -39,32 +39,27 @@ enum bpf_field_info_kind {
#endif
/*
* Extract bitfield, identified by src->field, and put its value into u64
* *res. All this is done in relocatable manner, so bitfield changes such as
* Extract bitfield, identified by s->field, and return its value as u64.
* All this is done in relocatable manner, so bitfield changes such as
* signedness, bit size, offset changes, this will be handled automatically.
* This version of macro is using bpf_probe_read() to read underlying integer
* storage. Macro functions as an expression and its return type is
* bpf_probe_read()'s return value: 0, on success, <0 on error.
*/
#define BPF_CORE_READ_BITFIELD_PROBED(src, field, res) ({ \
unsigned long long val; \
#define BPF_CORE_READ_BITFIELD_PROBED(s, field) ({ \
unsigned long long val = 0; \
\
*res = 0; \
val = __CORE_BITFIELD_PROBE_READ(res, src, field); \
if (!val) { \
*res <<= __CORE_RELO(src, field, LSHIFT_U64); \
val = __CORE_RELO(src, field, RSHIFT_U64); \
if (__CORE_RELO(src, field, SIGNED)) \
*res = ((long long)*res) >> val; \
else \
*res = ((unsigned long long)*res) >> val; \
val = 0; \
} \
__CORE_BITFIELD_PROBE_READ(&val, s, field); \
val <<= __CORE_RELO(s, field, LSHIFT_U64); \
if (__CORE_RELO(s, field, SIGNED)) \
val = ((long long)val) >> __CORE_RELO(s, field, RSHIFT_U64); \
else \
val = val >> __CORE_RELO(s, field, RSHIFT_U64); \
val; \
})
/*
* Extract bitfield, identified by src->field, and return its value as u64.
* Extract bitfield, identified by s->field, and return its value as u64.
* This version of macro is using direct memory reads and should be used from
* BPF program types that support such functionality (e.g., typed raw
* tracepoints).

19
tools/testing/selftests/bpf/progs/test_core_reloc_bitfields_probed.c
View file

@ -37,11 +37,6 @@ struct core_reloc_bitfields_output {
int64_t s32;
};
#define TRANSFER_BITFIELD(in, out, field) \
if (BPF_CORE_READ_BITFIELD_PROBED(in, field, &res)) \
return 1; \
out->field = res
SEC("raw_tracepoint/sys_enter")
int test_core_bitfields(void *ctx)
{
@ -49,13 +44,13 @@ int test_core_bitfields(void *ctx)
struct core_reloc_bitfields_output *out = (void *)&data.out;
uint64_t res;
TRANSFER_BITFIELD(in, out, ub1);
TRANSFER_BITFIELD(in, out, ub2);
TRANSFER_BITFIELD(in, out, ub7);
TRANSFER_BITFIELD(in, out, sb4);
TRANSFER_BITFIELD(in, out, sb20);
TRANSFER_BITFIELD(in, out, u32);
TRANSFER_BITFIELD(in, out, s32);
out->ub1 = BPF_CORE_READ_BITFIELD_PROBED(in, ub1);
out->ub2 = BPF_CORE_READ_BITFIELD_PROBED(in, ub2);
out->ub7 = BPF_CORE_READ_BITFIELD_PROBED(in, ub7);
out->sb4 = BPF_CORE_READ_BITFIELD_PROBED(in, sb4);
out->sb20 = BPF_CORE_READ_BITFIELD_PROBED(in, sb20);
out->u32 = BPF_CORE_READ_BITFIELD_PROBED(in, u32);
out->s32 = BPF_CORE_READ_BITFIELD_PROBED(in, s32);
return 0;
}