Messages in this thread |  | | Subject | Re: [PATCH v2 bpf-next 08/13] bpf: Add instructions for atomic_[cmp]xchg | From | Yonghong Song <> | Date | Fri, 27 Nov 2020 21:25:53 -0800 |
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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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