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

1 files changed, 17 insertions, 2 deletions

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;