[PATCH v2 bpf-next 10/13] bpf: Add instructions for atomic[64]_[fetch_]sub
From: Brendan Jackman
Date: Fri Nov 27 2020 - 12:58:24 EST
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@xxxxxxxxxx>
---
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