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authorBrendan Jackman <jackmanb@google.com>2021-01-14 18:17:47 +0000
committerAlexei Starovoitov <ast@kernel.org>2021-01-14 18:34:29 -0800
commit5ffa25502b5ab3d639829a2d1e316cff7f59a41e (patch)
treeb3ae91f0bdbcbad21f09cd5ed54b39ee7942c79f /kernel
parent5ca419f2864a2c60940dcf4bbaeb69546200e36f (diff)
downloadlinux-5ffa25502b5ab3d639829a2d1e316cff7f59a41e.tar.bz2
bpf: Add instructions for atomic_[cmp]xchg
This adds two atomic opcodes, both of which include the BPF_FETCH flag. XCHG without the BPF_FETCH flag would naturally encode 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, so that's what we use. Signed-off-by: Brendan Jackman <jackmanb@google.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Acked-by: Yonghong Song <yhs@fb.com> Link: https://lore.kernel.org/bpf/20210114181751.768687-8-jackmanb@google.com
Diffstat (limited to 'kernel')
-rw-r--r--kernel/bpf/core.c20
-rw-r--r--kernel/bpf/disasm.c15
-rw-r--r--kernel/bpf/verifier.c19
3 files changed, 52 insertions, 2 deletions
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 28d6000463e4..4df6daba43ef 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1630,6 +1630,16 @@ out:
(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 @@ out:
(u64) SRC,
(atomic64_t *)(unsigned long) (DST + insn->off));
break;
+ case BPF_XCHG:
+ SRC = (u64) atomic64_xchg(
+ (atomic64_t *)(unsigned long) (DST + insn->off),
+ (u64) SRC);
+ break;
+ case BPF_CMPXCHG:
+ BPF_R0 = (u64) atomic64_cmpxchg(
+ (atomic64_t *)(unsigned long) (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 d2e20f6d0516..ee8d1132767b 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",
+ 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",
+ 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 6aa1fc919761..89a4d154ab37 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3606,11 +3606,14 @@ 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 load_reg;
int err;
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);
@@ -3632,6 +3635,13 @@ 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;
@@ -3662,8 +3672,13 @@ 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;