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bpf: Add instructions for atomic_[cmp]xchg

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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
This commit is contained in:
Brendan Jackman 2021-01-14 18:17:47 +00:00 committed by Alexei Starovoitov
parent 5ca419f286
commit 5ffa25502b
8 changed files with 70 additions and 4 deletions
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8
arch/x86/net/bpf_jit_comp.c
View File

@ -815,6 +815,14 @@ static int emit_atomic(u8 **pprog, u8 atomic_op,
/* src_reg = atomic_fetch_add(dst_reg + off, src_reg); */
EMIT2(0x0F, 0xC1);
break;
case BPF_XCHG:
/* src_reg = atomic_xchg(dst_reg + off, src_reg); */
EMIT1(0x87);
break;
case BPF_CMPXCHG:
/* r0 = atomic_cmpxchg(dst_reg + off, r0, src_reg); */
EMIT2(0x0F, 0xB1);
break;
default:
pr_err("bpf_jit: unknown atomic opcode %02x\n", atomic_op);
return -EFAULT;

2
include/linux/filter.h
View File

@ -265,6 +265,8 @@ static inline bool insn_is_zext(const struct bpf_insn *insn)
*
* BPF_ADD *(uint *) (dst_reg + off16) += src_reg
* BPF_ADD | BPF_FETCH src_reg = atomic_fetch_add(dst_reg + off16, src_reg);
* BPF_XCHG src_reg = atomic_xchg(dst_reg + off16, src_reg)
* BPF_CMPXCHG r0 = atomic_cmpxchg(dst_reg + off16, r0, src_reg)
*/
#define BPF_ATOMIC_OP(SIZE, OP, DST, SRC, OFF) \

4
include/uapi/linux/bpf.h
View File

@ -45,7 +45,9 @@
#define BPF_EXIT 0x90 /* function return */
/* atomic op type fields (stored in immediate) */
#define BPF_FETCH 0x01 /* fetch previous value into src reg */
#define BPF_FETCH 0x01 /* not an opcode on its own, used to build others */
#define BPF_XCHG (0xe0 | BPF_FETCH) /* atomic exchange */
#define BPF_CMPXCHG (0xf0 | BPF_FETCH) /* atomic compare-and-write */
/* Register numbers */
enum {

20
kernel/bpf/core.c
View File

@ -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;
}

15
kernel/bpf/disasm.c
View File

@ -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);
}

19
kernel/bpf/verifier.c
View File

@ -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;

2
tools/include/linux/filter.h
View File

@ -174,6 +174,8 @@
*
* BPF_ADD *(uint *) (dst_reg + off16) += src_reg
* BPF_ADD | BPF_FETCH src_reg = atomic_fetch_add(dst_reg + off16, src_reg);
* BPF_XCHG src_reg = atomic_xchg(dst_reg + off16, src_reg)
* BPF_CMPXCHG r0 = atomic_cmpxchg(dst_reg + off16, r0, src_reg)
*/
#define BPF_ATOMIC_OP(SIZE, OP, DST, SRC, OFF) \

4
tools/include/uapi/linux/bpf.h
View File

@ -45,7 +45,9 @@
#define BPF_EXIT 0x90 /* function return */
/* atomic op type fields (stored in immediate) */
#define BPF_FETCH 0x01 /* fetch previous value into src reg */
#define BPF_FETCH 0x01 /* not an opcode on its own, used to build others */
#define BPF_XCHG (0xe0 | BPF_FETCH) /* atomic exchange */
#define BPF_CMPXCHG (0xf0 | BPF_FETCH) /* atomic compare-and-write */
/* Register numbers */
enum {