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    Subject[PATCH v2 bpf-next 10/13] bpf: Add instructions for atomic[64]_[fetch_]sub
    From
    Including only interpreter and x86 JIT support.

    x86 doesn't provide an atomic exchange-and-subtract instruction that
    could be used for BPF_SUB | BPF_FETCH, however we can just emit a NEG
    followed by an XADD to get the same effect.

    Signed-off-by: Brendan Jackman <jackmanb@google.com>
    ---
    arch/x86/net/bpf_jit_comp.c | 16 ++++++++++++++--
    include/linux/filter.h | 20 ++++++++++++++++++++
    kernel/bpf/core.c | 1 +
    kernel/bpf/disasm.c | 16 ++++++++++++----
    kernel/bpf/verifier.c | 2 ++
    tools/include/linux/filter.h | 20 ++++++++++++++++++++
    6 files changed, 69 insertions(+), 6 deletions(-)

    diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
    index 7431b2937157..a8a9fab13fcf 100644
    --- a/arch/x86/net/bpf_jit_comp.c
    +++ b/arch/x86/net/bpf_jit_comp.c
    @@ -823,6 +823,7 @@ static int emit_atomic(u8 **pprog, u8 atomic_op,

    /* emit opcode */
    switch (atomic_op) {
    + case BPF_SUB:
    case BPF_ADD:
    /* lock *(u32/u64*)(dst_reg + off) <op>= src_reg */
    EMIT1(simple_alu_opcodes[atomic_op]);
    @@ -1306,8 +1307,19 @@ st: if (is_imm8(insn->off))

    case BPF_STX | BPF_ATOMIC | BPF_W:
    case BPF_STX | BPF_ATOMIC | BPF_DW:
    - err = emit_atomic(&prog, insn->imm, dst_reg, src_reg,
    - insn->off, BPF_SIZE(insn->code));
    + if (insn->imm == (BPF_SUB | BPF_FETCH)) {
    + /*
    + * x86 doesn't have an XSUB insn, so we negate
    + * and XADD instead.
    + */
    + emit_neg(&prog, src_reg, BPF_SIZE(insn->code) == BPF_DW);
    + err = emit_atomic(&prog, BPF_ADD | BPF_FETCH,
    + dst_reg, src_reg, insn->off,
    + BPF_SIZE(insn->code));
    + } else {
    + err = emit_atomic(&prog, insn->imm, dst_reg, src_reg,
    + insn->off, BPF_SIZE(insn->code));
    + }
    if (err)
    return err;
    break;
    diff --git a/include/linux/filter.h b/include/linux/filter.h
    index 6186280715ed..a20a3a536bf5 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 memory sub, *(uint *)(dst_reg + off16) -= src_reg */
    +
    +#define BPF_ATOMIC_SUB(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_SUB })
    +
    +/* Atomic memory sub with fetch, src_reg = atomic_fetch_sub(*(dst_reg + off), src_reg); */
    +
    +#define BPF_ATOMIC_FETCH_SUB(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_SUB | BPF_FETCH })
    +
    /* Atomic exchange, src_reg = atomic_xchg((dst_reg + off), src_reg) */

    #define BPF_ATOMIC_XCHG(SIZE, DST, SRC, OFF) \
    diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
    index 20a5351d1dc2..0f700464955f 100644
    --- a/kernel/bpf/core.c
    +++ b/kernel/bpf/core.c
    @@ -1650,6 +1650,7 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
    STX_ATOMIC_W:
    switch (IMM) {
    ATOMIC(BPF_ADD, add)
    + ATOMIC(BPF_SUB, sub)

    case BPF_XCHG:
    if (BPF_SIZE(insn->code) == BPF_W)
    diff --git a/kernel/bpf/disasm.c b/kernel/bpf/disasm.c
    index 3441ac54ac65..f33acffdeed0 100644
    --- a/kernel/bpf/disasm.c
    +++ b/kernel/bpf/disasm.c
    @@ -80,6 +80,11 @@ const char *const bpf_alu_string[16] = {
    [BPF_END >> 4] = "endian",
    };

    +const char *const bpf_atomic_alu_string[16] = {
    + [BPF_ADD >> 4] = "add",
    + [BPF_SUB >> 4] = "sub",
    +};
    +
    static const char *const bpf_ldst_string[] = {
    [BPF_W >> 3] = "u32",
    [BPF_H >> 3] = "u16",
    @@ -154,17 +159,20 @@ void print_bpf_insn(const struct bpf_insn_cbs *cbs,
    insn->dst_reg,
    insn->off, insn->src_reg);
    else if (BPF_MODE(insn->code) == BPF_ATOMIC &&
    - insn->imm == BPF_ADD) {
    - verbose(cbs->private_data, "(%02x) lock *(%s *)(r%d %+d) += r%d\n",
    + (insn->imm == BPF_ADD || insn->imm == BPF_SUB)) {
    + verbose(cbs->private_data, "(%02x) lock *(%s *)(r%d %+d) %s r%d\n",
    insn->code,
    bpf_ldst_string[BPF_SIZE(insn->code) >> 3],
    insn->dst_reg, insn->off,
    + bpf_alu_string[BPF_OP(insn->imm) >> 4],
    insn->src_reg);
    } else if (BPF_MODE(insn->code) == BPF_ATOMIC &&
    - insn->imm == (BPF_ADD | BPF_FETCH)) {
    - verbose(cbs->private_data, "(%02x) r%d = atomic%s_fetch_add(*(%s *)(r%d %+d), r%d)\n",
    + (insn->imm == (BPF_ADD | BPF_FETCH) ||
    + insn->imm == (BPF_SUB | BPF_FETCH))) {
    + verbose(cbs->private_data, "(%02x) r%d = atomic%s_fetch_%s(*(%s *)(r%d %+d), r%d)\n",
    insn->code, insn->src_reg,
    BPF_SIZE(insn->code) == BPF_DW ? "64" : "",
    + bpf_atomic_alu_string[BPF_OP(insn->imm) >> 4],
    bpf_ldst_string[BPF_SIZE(insn->code) >> 3],
    insn->dst_reg, insn->off, insn->src_reg);
    } else if (BPF_MODE(insn->code) == BPF_ATOMIC &&
    diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
    index c8311cc114ec..dea9ad486ad1 100644
    --- a/kernel/bpf/verifier.c
    +++ b/kernel/bpf/verifier.c
    @@ -3606,6 +3606,8 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i
    switch (insn->imm) {
    case BPF_ADD:
    case BPF_ADD | BPF_FETCH:
    + case BPF_SUB:
    + case BPF_SUB | BPF_FETCH:
    case BPF_XCHG:
    case BPF_CMPXCHG:
    break;
    diff --git a/tools/include/linux/filter.h b/tools/include/linux/filter.h
    index ea99bd17d003..387eddaf11e5 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 memory sub, *(uint *)(dst_reg + off16) -= src_reg */
    +
    +#define BPF_ATOMIC_SUB(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_SUB })
    +
    +/* Atomic memory sub with fetch, src_reg = atomic_fetch_sub(*(dst_reg + off), src_reg); */
    +
    +#define BPF_ATOMIC_FETCH_SUB(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_SUB | BPF_FETCH })
    +
    /* Atomic exchange, src_reg = atomic_xchg((dst_reg + off), src_reg) */

    #define BPF_ATOMIC_XCHG(SIZE, DST, SRC, OFF) \
    --
    2.29.2.454.gaff20da3a2-goog
    \
     
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