On 5/5/23 8:30 AM, Alexei Starovoitov wrote:
On Thu, May 4, 2023 at 1:18 PM Yonghong Song <yhs@xxxxxxxx> wrote:
On 5/2/23 9:57 AM, Will Deacon wrote:
A narrow load from a 64-bit context field results in a 64-bit load
followed potentially by a 64-bit right-shift and then a bitwise AND
operation to extract the relevant data.
In the case of a 32-bit access, an immediate mask of 0xffffffff is used
to construct a 64-bit BPP_AND operation which then sign-extends the mask
value and effectively acts as a glorified no-op.
Fix the mask generation so that narrow loads always perform a 32-bit AND
operation.
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Cc: John Fastabend <john.fastabend@xxxxxxxxx>
Cc: Krzesimir Nowak <krzesimir@xxxxxxxxxx>
Cc: Yonghong Song <yhs@xxxxxx>
Cc: Andrey Ignatov <rdna@xxxxxx>
Fixes: 31fd85816dbe ("bpf: permits narrower load from bpf program context fields")
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
Thanks for the fix! You didn't miss anything. It is a bug and we did not
find it probably because user always use 'u64 val = ctx->u64_field' in
their bpf code...
But I think the commit message can be improved. An example to show the
difference without and with this patch can explain the issue much better.
Acked-by: Yonghong Song <yhs@xxxxxx>
If I'm reading it correctly it's indeed a bug.
alu64(and, 0xffffFFFF) is a nop
but it should have been
alu32(and, 0xffffFFFF) which will clear upper 32-bit, right?
Right. This is my understanding as well.
Would be good to have a selftest for this.
