[PATCH bpf-next v5 5/6] bpf, x86: Support load-acquire and store-release instructions

Peilin Ye
2025-03-04T00:14:27Z

Recently we introduced BPF load-acquire (BPF_LOAD_ACQ) and store-release
(BPF_STORE_REL) instructions. For x86-64, simply implement them as
regular BPF_LDX/BPF_STX loads and stores. The verifier always rejects
misaligned load-acquires/store-releases (even if BPF_F_ANY_ALIGNMENT is
set), so emitted MOV* instructions are guaranteed to be atomic.

Arena accesses are supported. 8- and 16-bit load-acquires are
zero-extending (i.e., MOVZBQ, MOVZWQ).

Rename emit_atomic{,_index}() to emit_atomic_rmw{,_index}() to make it
clear that they only handle read-modify-write atomics, and extend their
@atomic_op parameter from u8 to u32, since we are starting to use more
than the lowest 8 bits of the 'imm' field.

Signed-off-by: Peilin Ye <yepeilin@google.com>
---
arch/x86/net/bpf_jit_comp.c | 99 ++++++++++++++++++++++++++++++-------
1 file changed, 82 insertions(+), 17 deletions(-)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index f0c31c940fb8..0263d98d92b0 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -1242,8 +1242,8 @@ static void emit_st_r12(u8 **pprog, u32 size, u32 dst_reg, int off, int imm)
emit_st_index(pprog, size, dst_reg, X86_REG_R12, off, imm);
}

-static int emit_atomic(u8 **pprog, u8 atomic_op,
- u32 dst_reg, u32 src_reg, s16 off, u8 bpf_size)
+static int emit_atomic_rmw(u8 **pprog, u32 atomic_op,
+ u32 dst_reg, u32 src_reg, s16 off, u8 bpf_size)
{
u8 *prog = *pprog;

@@ -1283,8 +1283,9 @@ static int emit_atomic(u8 **pprog, u8 atomic_op,
return 0;
}

-static int emit_atomic_index(u8 **pprog, u8 atomic_op, u32 size,
- u32 dst_reg, u32 src_reg, u32 index_reg, int off)
+static int emit_atomic_rmw_index(u8 **pprog, u32 atomic_op, u32 size,
+ u32 dst_reg, u32 src_reg, u32 index_reg,
+ int off)
{
u8 *prog = *pprog;

@@ -1297,7 +1298,7 @@ static int emit_atomic_index(u8 **pprog, u8 atomic_op, u32 size,
EMIT1(add_3mod(0x48, dst_reg, src_reg, index_reg));
break;
default:
- pr_err("bpf_jit: 1 and 2 byte atomics are not supported\n");
+ pr_err("bpf_jit: 1- and 2-byte RMW atomics are not supported\n");
return -EFAULT;
}

@@ -1331,6 +1332,49 @@ static int emit_atomic_index(u8 **pprog, u8 atomic_op, u32 size,
return 0;
}

+static int emit_atomic_ld_st(u8 **pprog, u32 atomic_op, u32 dst_reg,
+ u32 src_reg, s16 off, u8 bpf_size)
+{
+ switch (atomic_op) {
+ case BPF_LOAD_ACQ:
+ /* dst_reg = smp_load_acquire(src_reg + off16) */
+ emit_ldx(pprog, bpf_size, dst_reg, src_reg, off);
+ break;
+ case BPF_STORE_REL:
+ /* smp_store_release(dst_reg + off16, src_reg) */
+ emit_stx(pprog, bpf_size, dst_reg, src_reg, off);
+ break;
+ default:
+ pr_err("bpf_jit: unknown atomic load/store opcode %02x\n",
+ atomic_op);
+ return -EFAULT;
+ }
+
+ return 0;
+}
+
+static int emit_atomic_ld_st_index(u8 **pprog, u32 atomic_op, u32 size,
+ u32 dst_reg, u32 src_reg, u32 index_reg,
+ int off)
+{
+ switch (atomic_op) {
+ case BPF_LOAD_ACQ:
+ /* dst_reg = smp_load_acquire(src_reg + idx_reg + off16) */
+ emit_ldx_index(pprog, size, dst_reg, src_reg, index_reg, off);
+ break;
+ case BPF_STORE_REL:
+ /* smp_store_release(dst_reg + idx_reg + off16, src_reg) */
+ emit_stx_index(pprog, size, dst_reg, src_reg, index_reg, off);
+ break;
+ default:
+ pr_err("bpf_jit: unknown atomic load/store opcode %02x\n",
+ atomic_op);
+ return -EFAULT;
+ }
+
+ return 0;
+}
+
#define DONT_CLEAR 1

bool ex_handler_bpf(const struct exception_table_entry *x, struct pt_regs *regs)
@@ -2113,6 +2157,13 @@ st: if (is_imm8(insn->off))
}
break;

+ case BPF_STX | BPF_ATOMIC | BPF_B:
+ case BPF_STX | BPF_ATOMIC | BPF_H:
+ if (!bpf_atomic_is_load_store(insn)) {
+ pr_err("bpf_jit: 1- and 2-byte RMW atomics are not supported\n");
+ return -EFAULT;
+ }
+ fallthrough;
case BPF_STX | BPF_ATOMIC | BPF_W:
case BPF_STX | BPF_ATOMIC | BPF_DW:
if (insn->imm == (BPF_AND | BPF_FETCH) ||
@@ -2148,10 +2199,10 @@ st: if (is_imm8(insn->off))
EMIT2(simple_alu_opcodes[BPF_OP(insn->imm)],
add_2reg(0xC0, AUX_REG, real_src_reg));
/* Attempt to swap in new value */
- err = emit_atomic(&prog, BPF_CMPXCHG,
- real_dst_reg, AUX_REG,
- insn->off,
- BPF_SIZE(insn->code));
+ err = emit_atomic_rmw(&prog, BPF_CMPXCHG,
+ real_dst_reg, AUX_REG,
+ insn->off,
+ BPF_SIZE(insn->code));
if (WARN_ON(err))
return err;
/*
@@ -2166,17 +2217,35 @@ st: if (is_imm8(insn->off))
break;
}

- err = emit_atomic(&prog, insn->imm, dst_reg, src_reg,
- insn->off, BPF_SIZE(insn->code));
+ if (bpf_atomic_is_load_store(insn))
+ err = emit_atomic_ld_st(&prog, insn->imm, dst_reg, src_reg,
+ insn->off, BPF_SIZE(insn->code));
+ else
+ err = emit_atomic_rmw(&prog, insn->imm, dst_reg, src_reg,
+ insn->off, BPF_SIZE(insn->code));
if (err)
return err;
break;

+ case BPF_STX | BPF_PROBE_ATOMIC | BPF_B:
+ case BPF_STX | BPF_PROBE_ATOMIC | BPF_H:
+ if (!bpf_atomic_is_load_store(insn)) {
+ pr_err("bpf_jit: 1- and 2-byte RMW atomics are not supported\n");
+ return -EFAULT;
+ }
+ fallthrough;
case BPF_STX | BPF_PROBE_ATOMIC | BPF_W:
case BPF_STX | BPF_PROBE_ATOMIC | BPF_DW:
start_of_ldx = prog;
- err = emit_atomic_index(&prog, insn->imm, BPF_SIZE(insn->code),
- dst_reg, src_reg, X86_REG_R12, insn->off);
+
+ if (bpf_atomic_is_load_store(insn))
+ err = emit_atomic_ld_st_index(&prog, insn->imm,
+ BPF_SIZE(insn->code), dst_reg,
+ src_reg, X86_REG_R12, insn->off);
+ else
+ err = emit_atomic_rmw_index(&prog, insn->imm, BPF_SIZE(insn->code),
+ dst_reg, src_reg, X86_REG_R12,
+ insn->off);
if (err)
return err;
goto populate_extable;
@@ -3771,12 +3840,8 @@ bool bpf_jit_supports_insn(struct bpf_insn *insn, bool in_arena)
if (!in_arena)
return true;
switch (insn->code) {
- case BPF_STX | BPF_ATOMIC | BPF_B:
- case BPF_STX | BPF_ATOMIC | BPF_H:
case BPF_STX | BPF_ATOMIC | BPF_W:
case BPF_STX | BPF_ATOMIC | BPF_DW:
- if (bpf_atomic_is_load_store(insn))
- return false;
if (insn->imm == (BPF_AND | BPF_FETCH) ||
insn->imm == (BPF_OR | BPF_FETCH) ||
insn->imm == (BPF_XOR | BPF_FETCH))