| Message ID | 20190730195408.670063-4-andriin@fb.com |
|---|---|
| State | Changes Requested |
| Delegated to: | BPF Maintainers |
| Headers | show |
| Series | CO-RE offset relocations | expand |
> On Jul 30, 2019, at 12:53 PM, Andrii Nakryiko <andriin@fb.com> wrote: > > Add BPF_CORE_READ macro used in tests to do bpf_core_read(), which > automatically captures offset relocation. > > Signed-off-by: Andrii Nakryiko <andriin@fb.com> > --- > tools/testing/selftests/bpf/bpf_helpers.h | 19 +++++++++++++++++++ > 1 file changed, 19 insertions(+) > > diff --git a/tools/testing/selftests/bpf/bpf_helpers.h b/tools/testing/selftests/bpf/bpf_helpers.h > index f804f210244e..81bc51293d11 100644 > --- a/tools/testing/selftests/bpf/bpf_helpers.h > +++ b/tools/testing/selftests/bpf/bpf_helpers.h > @@ -501,4 +501,23 @@ struct pt_regs; > (void *)(PT_REGS_FP(ctx) + sizeof(ip))); }) > #endif > > +/* > + * BPF_CORE_READ abstracts away bpf_probe_read() call and captures offset > + * relocation for source address using __builtin_preserve_access_index() > + * built-in, provided by Clang. > + * > + * __builtin_preserve_access_index() takes as an argument an expression of > + * taking an address of a field within struct/union. It makes compiler emit > + * a relocation, which records BTF type ID describing root struct/union and an > + * accessor string which describes exact embedded field that was used to take > + * an address. See detailed description of this relocation format and > + * semantics in comments to struct bpf_offset_reloc in libbpf_internal.h. > + * > + * This relocation allows libbpf to adjust BPF instruction to use correct > + * actual field offset, based on target kernel BTF type that matches original > + * (local) BTF, used to record relocation. > + */ > +#define BPF_CORE_READ(dst, src) \ > + bpf_probe_read(dst, sizeof(*src), __builtin_preserve_access_index(src)) We should use "sizeof(*(src))"
On Tue, Jul 30, 2019 at 2:24 PM Song Liu <songliubraving@fb.com> wrote: > > > > > On Jul 30, 2019, at 12:53 PM, Andrii Nakryiko <andriin@fb.com> wrote: > > > > Add BPF_CORE_READ macro used in tests to do bpf_core_read(), which > > automatically captures offset relocation. > > > > Signed-off-by: Andrii Nakryiko <andriin@fb.com> > > --- > > tools/testing/selftests/bpf/bpf_helpers.h | 19 +++++++++++++++++++ > > 1 file changed, 19 insertions(+) > > > > diff --git a/tools/testing/selftests/bpf/bpf_helpers.h b/tools/testing/selftests/bpf/bpf_helpers.h > > index f804f210244e..81bc51293d11 100644 > > --- a/tools/testing/selftests/bpf/bpf_helpers.h > > +++ b/tools/testing/selftests/bpf/bpf_helpers.h > > @@ -501,4 +501,23 @@ struct pt_regs; > > (void *)(PT_REGS_FP(ctx) + sizeof(ip))); }) > > #endif > > > > +/* > > + * BPF_CORE_READ abstracts away bpf_probe_read() call and captures offset > > + * relocation for source address using __builtin_preserve_access_index() > > + * built-in, provided by Clang. > > + * > > + * __builtin_preserve_access_index() takes as an argument an expression of > > + * taking an address of a field within struct/union. It makes compiler emit > > + * a relocation, which records BTF type ID describing root struct/union and an > > + * accessor string which describes exact embedded field that was used to take > > + * an address. See detailed description of this relocation format and > > + * semantics in comments to struct bpf_offset_reloc in libbpf_internal.h. > > + * > > + * This relocation allows libbpf to adjust BPF instruction to use correct > > + * actual field offset, based on target kernel BTF type that matches original > > + * (local) BTF, used to record relocation. > > + */ > > +#define BPF_CORE_READ(dst, src) \ > > + bpf_probe_read(dst, sizeof(*src), __builtin_preserve_access_index(src)) > > We should use "sizeof(*(src))" > Good point. Also (dst) instead of just (dst). Will update.
> On Jul 30, 2019, at 2:26 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote: > > On Tue, Jul 30, 2019 at 2:24 PM Song Liu <songliubraving@fb.com> wrote: >> >> >> >>> On Jul 30, 2019, at 12:53 PM, Andrii Nakryiko <andriin@fb.com> wrote: >>> >>> Add BPF_CORE_READ macro used in tests to do bpf_core_read(), which >>> automatically captures offset relocation. >>> >>> Signed-off-by: Andrii Nakryiko <andriin@fb.com> >>> --- >>> tools/testing/selftests/bpf/bpf_helpers.h | 19 +++++++++++++++++++ >>> 1 file changed, 19 insertions(+) >>> >>> diff --git a/tools/testing/selftests/bpf/bpf_helpers.h b/tools/testing/selftests/bpf/bpf_helpers.h >>> index f804f210244e..81bc51293d11 100644 >>> --- a/tools/testing/selftests/bpf/bpf_helpers.h >>> +++ b/tools/testing/selftests/bpf/bpf_helpers.h >>> @@ -501,4 +501,23 @@ struct pt_regs; >>> (void *)(PT_REGS_FP(ctx) + sizeof(ip))); }) >>> #endif >>> >>> +/* >>> + * BPF_CORE_READ abstracts away bpf_probe_read() call and captures offset >>> + * relocation for source address using __builtin_preserve_access_index() >>> + * built-in, provided by Clang. >>> + * >>> + * __builtin_preserve_access_index() takes as an argument an expression of >>> + * taking an address of a field within struct/union. It makes compiler emit >>> + * a relocation, which records BTF type ID describing root struct/union and an >>> + * accessor string which describes exact embedded field that was used to take >>> + * an address. See detailed description of this relocation format and >>> + * semantics in comments to struct bpf_offset_reloc in libbpf_internal.h. >>> + * >>> + * This relocation allows libbpf to adjust BPF instruction to use correct >>> + * actual field offset, based on target kernel BTF type that matches original >>> + * (local) BTF, used to record relocation. >>> + */ >>> +#define BPF_CORE_READ(dst, src) \ >>> + bpf_probe_read(dst, sizeof(*src), __builtin_preserve_access_index(src)) >> >> We should use "sizeof(*(src))" >> > > Good point. Also (dst) instead of just (dst). Will update. I think dst as-is is fine. "," is the very last in precedence list.
diff --git a/tools/testing/selftests/bpf/bpf_helpers.h b/tools/testing/selftests/bpf/bpf_helpers.h index f804f210244e..81bc51293d11 100644 --- a/tools/testing/selftests/bpf/bpf_helpers.h +++ b/tools/testing/selftests/bpf/bpf_helpers.h @@ -501,4 +501,23 @@ struct pt_regs; (void *)(PT_REGS_FP(ctx) + sizeof(ip))); }) #endif +/* + * BPF_CORE_READ abstracts away bpf_probe_read() call and captures offset + * relocation for source address using __builtin_preserve_access_index() + * built-in, provided by Clang. + * + * __builtin_preserve_access_index() takes as an argument an expression of + * taking an address of a field within struct/union. It makes compiler emit + * a relocation, which records BTF type ID describing root struct/union and an + * accessor string which describes exact embedded field that was used to take + * an address. See detailed description of this relocation format and + * semantics in comments to struct bpf_offset_reloc in libbpf_internal.h. + * + * This relocation allows libbpf to adjust BPF instruction to use correct + * actual field offset, based on target kernel BTF type that matches original + * (local) BTF, used to record relocation. + */ +#define BPF_CORE_READ(dst, src) \ + bpf_probe_read(dst, sizeof(*src), __builtin_preserve_access_index(src)) + #endif
Add BPF_CORE_READ macro used in tests to do bpf_core_read(), which automatically captures offset relocation. Signed-off-by: Andrii Nakryiko <andriin@fb.com> --- tools/testing/selftests/bpf/bpf_helpers.h | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+)