Re: [PATCH bpf-next v3 08/14] bpf: Add instructions for atomic_[cmp]xchg

From: Brendan Jackman
Date: Fri Dec 04 2020 - 04:27:12 EST

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O Thu, Dec 03, 2020 at 09:34:23PM -0800, Yonghong Song wrote:
> On 12/3/20 8:02 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 => BPF_FETCH

Thanks, sorry I think you've already pointed that one out and I didn't fix it!

> > 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@xxxxxxxxxx>
>
> Ack with minor comments in the above and below.

Thanks, ack to all the comments.

Have run a `grep -r "atomic_.*(\*" *.patch` - hopefully we're now free
of this mistake where the first arg is dereferenced in the
comments/disasm...

> Acked-by: Yonghong Song <yhs@xxxxxx>
>
> > Change-Id: I3f19ad867dfd08515eecf72674e6fdefe28424bb
> > ---
> > 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/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> > index 88cb09fa3bfb..7d29bc3bb4ff 100644
> > --- a/arch/x86/net/bpf_jit_comp.c
> > +++ b/arch/x86/net/bpf_jit_comp.c
> > @@ -831,6 +831,14 @@ static int emit_atomic(u8 **pprog, u8 atomic_op,
> > /* src_reg = atomic_fetch_add(*(dst_reg + off), src_reg); */
> > EMIT2(0x0F, 0xC1);
> > break;
> > + case BPF_XCHG:
> > + /* src_reg = atomic_xchg(*(u32/u64*)(dst_reg + off), src_reg); */
>
> src_reg = atomic_xchg((u32/u64*)(dst_reg + off), src_reg)?
>
> > + EMIT1(0x87);
> > + break;
> > + case BPF_CMPXCHG:
> > + /* r0 = atomic_cmpxchg(*(u32/u64*)(dst_reg + off), r0, src_reg); */
>
> r0 = atomic_cmpxchg((u32/u64*)(dst_reg + off), r0, src_reg)?
>
> > + EMIT2(0x0F, 0xB1);
> > + break;
> > default:
> > pr_err("bpf_jit: unknown atomic opcode %02x\n", atomic_op);
> > return -EFAULT;
> > diff --git a/include/linux/filter.h b/include/linux/filter.h
> > index 4e04d0fc454f..6186280715ed 100644
> > --- a/include/linux/filter.h
> > +++ b/include/linux/filter.h
> > @@ -280,6 +280,26 @@ static inline bool insn_is_zext(const struct bpf_insn *insn)
> > .off = OFF, \
> > .imm = BPF_ADD | BPF_FETCH })
> > +/* Atomic exchange, src_reg = atomic_xchg((dst_reg + off), src_reg) */
>
> src_reg = atomic_xchg(dst_reg + off, src_reg)?
>
> > +
> > +#define BPF_ATOMIC_XCHG(SIZE, DST, SRC, OFF) \
> > + ((struct bpf_insn) { \
> > + .code = BPF_STX | BPF_SIZE(SIZE) | BPF_ATOMIC, \
> > + .dst_reg = DST, \
> > + .src_reg = SRC, \
> > + .off = OFF, \
> > + .imm = BPF_XCHG })
> > +
> > +/* Atomic compare-exchange, r0 = atomic_cmpxchg((dst_reg + off), r0, src_reg) */
>
> r0 = atomic_cmpxchg(dst_reg + off, r0, src_reg)?
>
> > +
> > +#define BPF_ATOMIC_CMPXCHG(SIZE, DST, SRC, OFF) \
> > + ((struct bpf_insn) { \
> > + .code = BPF_STX | BPF_SIZE(SIZE) | BPF_ATOMIC, \
> > + .dst_reg = DST, \
> > + .src_reg = SRC, \
> > + .off = OFF, \
> > + .imm = BPF_CMPXCHG })
> > +
> > /* Memory store, *(uint *) (dst_reg + off16) = imm32 */
> > #define BPF_ST_MEM(SIZE, DST, OFF, IMM) \
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index 025e377e7229..53334530cc81 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -45,7 +45,9 @@
> > #define BPF_EXIT 0x90 /* function return */
> > /* atomic op type fields (stored in immediate) */
> > -#define BPF_FETCH 0x01 /* fetch previous value into src reg */
> > +#define BPF_XCHG (0xe0 | BPF_FETCH) /* atomic exchange */
> > +#define BPF_CMPXCHG (0xf0 | BPF_FETCH) /* atomic compare-and-write */
> > +#define BPF_FETCH 0x01 /* not an opcode on its own, used to build others */
> > /* Register numbers */
> > enum {
> > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> > index 61e93eb7d363..28f960bc2e30 100644
> > --- a/kernel/bpf/core.c
> > +++ b/kernel/bpf/core.c
> > @@ -1630,6 +1630,16 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
> > (u32) SRC,
> > (atomic_t *)(unsigned long) (DST + insn->off));
> > break;
> > + case BPF_XCHG:
> > + SRC = (u32) atomic_xchg(
> > + (atomic_t *)(unsigned long) (DST + insn->off),
> > + (u32) SRC);
> > + break;
> > + case BPF_CMPXCHG:
> > + BPF_R0 = (u32) atomic_cmpxchg(
> > + (atomic_t *)(unsigned long) (DST + insn->off),
> > + (u32) BPF_R0, (u32) SRC);
> > + break;
> > default:
> > goto default_label;
> > }
> > @@ -1647,6 +1657,16 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
> > (u64) SRC,
> > (atomic64_t *)(s64) (DST + insn->off));
> > break;
> > + case BPF_XCHG:
> > + SRC = (u64) atomic64_xchg(
> > + (atomic64_t *)(u64) (DST + insn->off),
> > + (u64) SRC);
> > + break;
> > + case BPF_CMPXCHG:
> > + BPF_R0 = (u64) atomic64_cmpxchg(
> > + (atomic64_t *)(u64) (DST + insn->off),
> > + (u64) BPF_R0, (u64) SRC);
> > + break;
> > default:
> > goto default_label;
> > }
> > diff --git a/kernel/bpf/disasm.c b/kernel/bpf/disasm.c
> > index 3ee2246a52ef..18357ea9a17d 100644
> > --- a/kernel/bpf/disasm.c
> > +++ b/kernel/bpf/disasm.c
> > @@ -167,6 +167,21 @@ void print_bpf_insn(const struct bpf_insn_cbs *cbs,
> > BPF_SIZE(insn->code) == BPF_DW ? "64" : "",
> > bpf_ldst_string[BPF_SIZE(insn->code) >> 3],
> > insn->dst_reg, insn->off, insn->src_reg);
> > + } else if (BPF_MODE(insn->code) == BPF_ATOMIC &&
> > + insn->imm == BPF_CMPXCHG) {
> > + verbose(cbs->private_data, "(%02x) r0 = atomic%s_cmpxchg(*(%s *)(r%d %+d), r0, r%d)\n",
>
> (%02x) r0 = atomic%s_cmpxchg((%s *)(r%d %+d), r0, r%d)?
>
> > + insn->code,
> > + BPF_SIZE(insn->code) == BPF_DW ? "64" : "",
> > + bpf_ldst_string[BPF_SIZE(insn->code) >> 3],
> > + insn->dst_reg, insn->off,
> > + insn->src_reg);
> > + } else if (BPF_MODE(insn->code) == BPF_ATOMIC &&
> > + insn->imm == BPF_XCHG) {
> > + verbose(cbs->private_data, "(%02x) r%d = atomic%s_xchg(*(%s *)(r%d %+d), r%d)\n",
>
> (%02x) r%d = atomic%s_xchg((%s *)(r%d %+d), r%d)?
>
> > + insn->code, insn->src_reg,
> > + BPF_SIZE(insn->code) == BPF_DW ? "64" : "",
> > + bpf_ldst_string[BPF_SIZE(insn->code) >> 3],
> > + insn->dst_reg, insn->off, insn->src_reg);
> > } else {
> > verbose(cbs->private_data, "BUG_%02x\n", insn->code);
> > }
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index a68adbcee370..ccf4315e54e7 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;
>
> nit: not a big deal but maybe put this definition before 'int err' to
> maintain reverse christmas tree coding style.
>
> > 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 comparison of R0 with memory location */
> > + 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;
> > diff --git a/tools/include/linux/filter.h b/tools/include/linux/filter.h
> > index ac7701678e1a..ea99bd17d003 100644
> > --- a/tools/include/linux/filter.h
> > +++ b/tools/include/linux/filter.h
> > @@ -190,6 +190,26 @@
> > .off = OFF, \
> > .imm = BPF_ADD | BPF_FETCH })
> > +/* Atomic exchange, src_reg = atomic_xchg((dst_reg + off), src_reg) */
>
> src_reg = atomic_xchg(dst_reg + off, src_reg)?
>
> > +
> > +#define BPF_ATOMIC_XCHG(SIZE, DST, SRC, OFF) \
> > + ((struct bpf_insn) { \
> > + .code = BPF_STX | BPF_SIZE(SIZE) | BPF_ATOMIC, \
> > + .dst_reg = DST, \
> > + .src_reg = SRC, \
> > + .off = OFF, \
> > + .imm = BPF_XCHG })
> > +
> > +/* Atomic compare-exchange, r0 = atomic_cmpxchg((dst_reg + off), r0, src_reg) */
>
> r0 = atomic_cmpxchg(dst_reg + off, r0, src_reg)?
>
> > +
> > +#define BPF_ATOMIC_CMPXCHG(SIZE, DST, SRC, OFF) \
> > + ((struct bpf_insn) { \
> > + .code = BPF_STX | BPF_SIZE(SIZE) | BPF_ATOMIC, \
> > + .dst_reg = DST, \
> > + .src_reg = SRC, \
> > + .off = OFF, \
> > + .imm = BPF_CMPXCHG })
> > +
> > /* Memory store, *(uint *) (dst_reg + off16) = imm32 */
> > #define BPF_ST_MEM(SIZE, DST, OFF, IMM) \
> > diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> > index 025e377e7229..53334530cc81 100644
> > --- a/tools/include/uapi/linux/bpf.h
> > +++ b/tools/include/uapi/linux/bpf.h
> > @@ -45,7 +45,9 @@
> > #define BPF_EXIT 0x90 /* function return */
> > /* atomic op type fields (stored in immediate) */
> > -#define BPF_FETCH 0x01 /* fetch previous value into src reg */
> > +#define BPF_XCHG (0xe0 | BPF_FETCH) /* atomic exchange */
> > +#define BPF_CMPXCHG (0xf0 | BPF_FETCH) /* atomic compare-and-write */
> > +#define BPF_FETCH 0x01 /* not an opcode on its own, used to build others */
> > /* Register numbers */
> > enum {
> >

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Previous message: Linus Walleij: "Re: [PATCH 2/2] pinctrl: qcom: Add SM8350 pinctrl driver"
In reply to: Yonghong Song: "Re: [PATCH bpf-next v3 08/14] bpf: Add instructions for atomic_[cmp]xchg"
Next in thread: Brendan Jackman: "[PATCH bpf-next v3 04/14] bpf: x86: Factor out a lookup table for some ALU opcodes"
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