| Message ID | 20191221062604.1182843-1-kafai@fb.com |
|---|---|
| State | Changes Requested |
| Delegated to: | BPF Maintainers |
| Headers | show |
| Series | Introduce BPF STRUCT_OPS | expand |
On 12/20/19 10:26 PM, Martin KaFai Lau wrote: > This patch allows bitfield access as a scalar. > > Signed-off-by: Martin KaFai Lau <kafai@fb.com> Acked-by: Yonghong Song <yhs@fb.com>
On Fri, Dec 20, 2019 at 10:26 PM Martin KaFai Lau <kafai@fb.com> wrote: > > This patch allows bitfield access as a scalar. > > Signed-off-by: Martin KaFai Lau <kafai@fb.com> > --- > kernel/bpf/btf.c | 10 ++++++---- > 1 file changed, 6 insertions(+), 4 deletions(-) > > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c > index 6e652643849b..da73b63acfc5 100644 > --- a/kernel/bpf/btf.c > +++ b/kernel/bpf/btf.c > @@ -3744,10 +3744,6 @@ int btf_struct_access(struct bpf_verifier_log *log, > } > > for_each_member(i, t, member) { > - if (btf_member_bitfield_size(t, member)) > - /* bitfields are not supported yet */ > - continue; > - > /* offset of the field in bytes */ > moff = btf_member_bit_offset(t, member) / 8; > if (off + size <= moff) > @@ -3757,6 +3753,12 @@ int btf_struct_access(struct bpf_verifier_log *log, > if (off < moff) > continue; > > + if (btf_member_bitfield_size(t, member)) { > + if (off == moff && off + size <= t->size) > + return SCALAR_VALUE; > + continue; > + } Shouldn't this check be done before (off < moff) above? Imagine this situation: struct { int :16; int x:8; }; Compiler will generate 4-byte load with offset 0, and then bit shifts to extract third byte. From kernel perspective, you'll see that off=0, but moff=2, which will get skipped. So there are two problems, I think: 1. if member is bitfield, special handle that before (off < moff) case. 2. off == moff is too precise, I think it should be `off <= moff`, but also check that it covers entire bitfield, e.g.: (off + size) * 8 >= btf_member_bit_offset(t, member) + btf_member_bitfield_size(t, member) Make sense or am I missing anything? > + > /* type of the field */ > mtype = btf_type_by_id(btf_vmlinux, member->type); > mname = __btf_name_by_offset(btf_vmlinux, member->name_off); > -- > 2.17.1 >
On 12/23/19 12:05 PM, Andrii Nakryiko wrote: > On Fri, Dec 20, 2019 at 10:26 PM Martin KaFai Lau <kafai@fb.com> wrote: >> >> This patch allows bitfield access as a scalar. >> >> Signed-off-by: Martin KaFai Lau <kafai@fb.com> >> --- >> kernel/bpf/btf.c | 10 ++++++---- >> 1 file changed, 6 insertions(+), 4 deletions(-) >> >> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c >> index 6e652643849b..da73b63acfc5 100644 >> --- a/kernel/bpf/btf.c >> +++ b/kernel/bpf/btf.c >> @@ -3744,10 +3744,6 @@ int btf_struct_access(struct bpf_verifier_log *log, >> } >> >> for_each_member(i, t, member) { >> - if (btf_member_bitfield_size(t, member)) >> - /* bitfields are not supported yet */ >> - continue; >> - >> /* offset of the field in bytes */ >> moff = btf_member_bit_offset(t, member) / 8; >> if (off + size <= moff) >> @@ -3757,6 +3753,12 @@ int btf_struct_access(struct bpf_verifier_log *log, >> if (off < moff) >> continue; >> >> + if (btf_member_bitfield_size(t, member)) { >> + if (off == moff && off + size <= t->size) >> + return SCALAR_VALUE; >> + continue; >> + } > > Shouldn't this check be done before (off < moff) above? > > Imagine this situation: > > struct { > int :16; > int x:8; > }; Oh, yes, forgot the case where the first bitfield member may have no name, in which case, `off` will not match any `moff`. btf_struct_access is used to check vmlinux btf types. I think in vmlinux we may not have such scenarios. So the above code should handle vmlinux use cases properly. But I agree with Andrii that we probably want to handle anonymous bitfield member (which is ignored in debuginfo and BTF) properly. > > Compiler will generate 4-byte load with offset 0, and then bit shifts > to extract third byte. From kernel perspective, you'll see that off=0, > but moff=2, which will get skipped. > > So there are two problems, I think: > 1. if member is bitfield, special handle that before (off < moff) case. > 2. off == moff is too precise, I think it should be `off <= moff`, but > also check that it covers entire bitfield, e.g.: > > (off + size) * 8 >= btf_member_bit_offset(t, member) + > btf_member_bitfield_size(t, member) > > Make sense or am I missing anything? > >> + >> /* type of the field */ >> mtype = btf_type_by_id(btf_vmlinux, member->type); >> mname = __btf_name_by_offset(btf_vmlinux, member->name_off); >> -- >> 2.17.1 >>
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c index 6e652643849b..da73b63acfc5 100644 --- a/kernel/bpf/btf.c +++ b/kernel/bpf/btf.c @@ -3744,10 +3744,6 @@ int btf_struct_access(struct bpf_verifier_log *log, } for_each_member(i, t, member) { - if (btf_member_bitfield_size(t, member)) - /* bitfields are not supported yet */ - continue; - /* offset of the field in bytes */ moff = btf_member_bit_offset(t, member) / 8; if (off + size <= moff) @@ -3757,6 +3753,12 @@ int btf_struct_access(struct bpf_verifier_log *log, if (off < moff) continue; + if (btf_member_bitfield_size(t, member)) { + if (off == moff && off + size <= t->size) + return SCALAR_VALUE; + continue; + } + /* type of the field */ mtype = btf_type_by_id(btf_vmlinux, member->type); mname = __btf_name_by_offset(btf_vmlinux, member->name_off);
This patch allows bitfield access as a scalar. Signed-off-by: Martin KaFai Lau <kafai@fb.com> --- kernel/bpf/btf.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-)