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SubjectRe: [PATCH v2 bpf-next 08/13] bpf: Add instructions for atomic_[cmp]xchg
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On 11/27/20 9:57 AM, Brendan Jackman wrote:
> This adds two atomic opcodes, both of which include the BPF_FETCH
> flag. XCHG without the BPF_FETCh flag would naturally encode

BPF_FETCH

> atomic_set. This is not supported because it would be of limited
> value to userspace (it doesn't imply any barriers). CMPXCHG without
> BPF_FETCH woulud be an atomic compare-and-write. We don't have such
> an operation in the kernel so it isn't provided to BPF either.
>
> There are two significant design decisions made for the CMPXCHG
> instruction:
>
> - To solve the issue that this operation fundamentally has 3
> operands, but we only have two register fields. Therefore the
> operand we compare against (the kernel's API calls it 'old') is
> hard-coded to be R0. x86 has similar design (and A64 doesn't
> have this problem).
>
> A potential alternative might be to encode the other operand's
> register number in the immediate field.
>
> - The kernel's atomic_cmpxchg returns the old value, while the C11
> userspace APIs return a boolean indicating the comparison
> result. Which should BPF do? A64 returns the old value. x86 returns
> the old value in the hard-coded register (and also sets a
> flag). That means return-old-value is easier to JIT.
>
> Signed-off-by: Brendan Jackman <jackmanb@google.com>
> ---
> arch/x86/net/bpf_jit_comp.c | 8 ++++++++
> include/linux/filter.h | 20 ++++++++++++++++++++
> include/uapi/linux/bpf.h | 4 +++-
> kernel/bpf/core.c | 20 ++++++++++++++++++++
> kernel/bpf/disasm.c | 15 +++++++++++++++
> kernel/bpf/verifier.c | 19 +++++++++++++++++--
> tools/include/linux/filter.h | 20 ++++++++++++++++++++
> tools/include/uapi/linux/bpf.h | 4 +++-
> 8 files changed, 106 insertions(+), 4 deletions(-)
>
[...]
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index cd4c03b25573..c8311cc114ec 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -3601,10 +3601,13 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
> static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_insn *insn)
> {
> int err;
> + int load_reg;
>
> switch (insn->imm) {
> case BPF_ADD:
> case BPF_ADD | BPF_FETCH:
> + case BPF_XCHG:
> + case BPF_CMPXCHG:
> break;
> default:
> verbose(env, "BPF_ATOMIC uses invalid atomic opcode %02x\n", insn->imm);
> @@ -3626,6 +3629,13 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i
> if (err)
> return err;
>
> + if (insn->imm == BPF_CMPXCHG) {
> + /* check src3 operand */

better comment about what src3 means here?

> + err = check_reg_arg(env, BPF_REG_0, SRC_OP);
> + if (err)
> + return err;
> + }
> +
> if (is_pointer_value(env, insn->src_reg)) {
> verbose(env, "R%d leaks addr into mem\n", insn->src_reg);
> return -EACCES;
> @@ -3656,8 +3666,13 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i
> 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);
> + if (insn->imm == BPF_CMPXCHG)
> + load_reg = BPF_REG_0;
> + else
> + load_reg = insn->src_reg;
> +
> + /* check and record load of old value */
> + err = check_reg_arg(env, load_reg, DST_OP);
> if (err)
> return err;
>
[...]

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