On 11/6/19 12:15 PM, Andrii Nakryiko wrote:
> 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.
Agreed. This will be consistent with direct read where
returning value will be 0 if any fault happens.
In really rare cases, if user want to distinguish good value 0 from
bpf_probe_read() returning error, all building macros are in the header
file, user can have a custom solution. But let us have API work
for common use case with good usability.
>
> Cc: Yonghong Song <yhs@xxxxxx>
> Signed-off-by: Andrii Nakryiko <andriin@xxxxxx>
Acked-by: Yonghong Song <yhs@xxxxxx>
> ---
> tools/lib/bpf/bpf_core_read.h | 27 ++++++++-----------
> .../progs/test_core_reloc_bitfields_probed.c | 19 +++++--------
> 2 files changed, 18 insertions(+), 28 deletions(-)
>
> diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h
> index 11461b2623b0..7009dc90e012 100644
> --- a/tools/lib/bpf/bpf_core_read.h
> +++ b/tools/lib/bpf/bpf_core_read.h
> @@ -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).
> diff --git a/tools/testing/selftests/bpf/progs/test_core_reloc_bitfields_probed.c b/tools/testing/selftests/bpf/progs/test_core_reloc_bitfields_probed.c
> index a381f8ac2419..e466e3ab7de4 100644
> --- a/tools/testing/selftests/bpf/progs/test_core_reloc_bitfields_probed.c
> +++ b/tools/testing/selftests/bpf/progs/test_core_reloc_bitfields_probed.c
> @@ -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;
> }
>
