On Mon, Dec 07, 2020 at 09:31:40PM -0800, John Fastabend wrote:
> Brendan Jackman wrote:
> > The BPF_FETCH field can be set in bpf_insn.imm, for BPF_ATOMIC
> > instructions, in order to have the previous value of the
> > atomically-modified memory location loaded into the src register
> > after an atomic op is carried out.
> >
> > Suggested-by: Yonghong Song <y...@fb.com>
> > Signed-off-by: Brendan Jackman <jackm...@google.com>
> > ---
>
> I like Yonghong suggestion
>
> #define BPF_ATOMIC_FETCH_ADD(SIZE, DST, SRC, OFF) \
> BPF_ATOMIC(SIZE, DST, SRC, OFF, BPF_ADD | BPF_FETCH)
>
> otherwise LGTM. One observation to consider below.
>
> Acked-by: John Fastabend <john.fastab...@gmail.com>
>
> > arch/x86/net/bpf_jit_comp.c | 4 ++++
> > include/linux/filter.h | 1 +
> > include/uapi/linux/bpf.h | 3 +++
> > kernel/bpf/core.c | 13 +++++++++++++
> > kernel/bpf/disasm.c | 7 +++++++
> > kernel/bpf/verifier.c | 33 ++++++++++++++++++++++++---------
> > tools/include/linux/filter.h | 11 +++++++++++
> > tools/include/uapi/linux/bpf.h | 3 +++
> > 8 files changed, 66 insertions(+), 9 deletions(-)
>
> [...]
>
> > @@ -3652,8 +3656,20 @@ static int check_atomic(struct bpf_verifier_env
> > *env, int insn_idx, struct bpf_i
> > return err;
> >
> > /* check whether we can write into the same memory */
> > - return check_mem_access(env, insn_idx, insn->dst_reg, insn->off,
> > - BPF_SIZE(insn->code), BPF_WRITE, -1, true);
> > + err = check_mem_access(env, insn_idx, insn->dst_reg, insn->off,
> > + BPF_SIZE(insn->code), BPF_WRITE, -1, true);
> > + if (err)
> > + return err;
> > +
> > + if (!(insn->imm & BPF_FETCH))
> > + return 0;
> > +
> > + /* check and record load of old value into src reg */
> > + err = check_reg_arg(env, insn->src_reg, DST_OP);
>
> This will mark the reg unknown. I think this is fine here. Might be nice
> to carry bounds through though if possible
Ah, I hadn't thought of this. I think if I move this check_reg_arg to be
before the first check_mem_access, and then (when BPF_FETCH) set the
val_regno arg to load_reg, then the bounds from memory would get
propagated back to the register:
if (insn->imm & BPF_FETCH) {
if (insn->imm == BPF_CMPXCHG)
load_reg = BPF_REG_0;
else
load_reg = insn->src_reg;
err = check_reg_arg(env, load_reg, DST_OP);
if (err)
return err;
} else {
load_reg = -1;
}
/* check wether we can read the memory */
err = check_mem_access(env, insn_index, insn->dst_reg, insn->off
BPF_SIZE(insn->code), BPF_READ,
load_reg, // <--
true);
Is that the kind of thing you had in mind?
> > + if (err)
> > + return err;
> > +
> > + return 0;
> > }
> >
