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Message-Id: <20201127175738.1085417-9-jackmanb@google.com>
Date: Fri, 27 Nov 2020 17:57:33 +0000
From: Brendan Jackman <jackmanb@...gle.com>
To: bpf@...r.kernel.org
Cc: Alexei Starovoitov <ast@...nel.org>, Yonghong Song <yhs@...com>,
Daniel Borkmann <daniel@...earbox.net>,
KP Singh <kpsingh@...omium.org>,
Florent Revest <revest@...omium.org>,
linux-kernel@...r.kernel.org, Jann Horn <jannh@...gle.com>,
Brendan Jackman <jackmanb@...gle.com>
Subject: [PATCH v2 bpf-next 08/13] 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.
Signed-off-by: Brendan Jackman <jackmanb@...gle.com>
---
arch/x86/net/bpf_jit_comp.c | 8 ++++++++
include/linux/filter.h | 20 ++++++++++++++++++++
include/uapi/linux/bpf.h | 4 +++-
kernel/bpf/core.c | 20 ++++++++++++++++++++
kernel/bpf/disasm.c | 15 +++++++++++++++
kernel/bpf/verifier.c | 19 +++++++++++++++++--
tools/include/linux/filter.h | 20 ++++++++++++++++++++
tools/include/uapi/linux/bpf.h | 4 +++-
8 files changed, 106 insertions(+), 4 deletions(-)
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index d3cd45bcd0c1..7431b2937157 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -831,6 +831,14 @@ 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(*(u32/u64*)(dst_reg + off), src_reg); */
+ EMIT1(0x87);
+ break;
+ case BPF_CMPXCHG:
+ /* r0 = atomic_cmpxchg(*(u32/u64*)(dst_reg + off), r0, src_reg); */
+ EMIT2(0x0F, 0xB1);
+ break;
default:
pr_err("bpf_jit: unknown atomic opcode %02x\n", atomic_op);
return -EFAULT;
diff --git a/include/linux/filter.h b/include/linux/filter.h
index 4e04d0fc454f..6186280715ed 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 exchange, src_reg = atomic_xchg((dst_reg + off), src_reg) */
+
+#define BPF_ATOMIC_XCHG(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_XCHG })
+
+/* Atomic compare-exchange, r0 = atomic_cmpxchg((dst_reg + off), r0, src_reg) */
+
+#define BPF_ATOMIC_CMPXCHG(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_CMPXCHG })
+
/* Memory store, *(uint *) (dst_reg + off16) = imm32 */
#define BPF_ST_MEM(SIZE, DST, OFF, IMM) \
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 025e377e7229..82039a1176ac 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -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_XCHG (0xe0 | BPF_FETCH) /* atomic exchange */
+#define BPF_CMPXCHG (0xf0 | BPF_FETCH) /* atomic compare-and-write */
+#define BPF_FETCH 0x01 /* fetch previous value into src reg or r0*/
/* Register numbers */
enum {
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 49a2a533db60..05350a8f87c0 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1638,6 +1638,16 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
(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;
}
@@ -1655,6 +1665,16 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
(u64) SRC,
(atomic64_t *)(s64) (DST + insn->off));
break;
+ case BPF_XCHG:
+ SRC = (u64) atomic64_xchg(
+ (atomic64_t *)(u64) (DST + insn->off),
+ (u64) SRC);
+ break;
+ case BPF_CMPXCHG:
+ BPF_R0 = (u64) atomic64_cmpxchg(
+ (atomic64_t *)(u64) (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 3ee2246a52ef..3441ac54ac65 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 cd4c03b25573..c8311cc114ec 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3601,10 +3601,13 @@ 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 err;
+ int load_reg;
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);
@@ -3626,6 +3629,13 @@ 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 src3 operand */
+ 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;
@@ -3656,8 +3666,13 @@ 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;
diff --git a/tools/include/linux/filter.h b/tools/include/linux/filter.h
index ac7701678e1a..ea99bd17d003 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 exchange, src_reg = atomic_xchg((dst_reg + off), src_reg) */
+
+#define BPF_ATOMIC_XCHG(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_XCHG })
+
+/* Atomic compare-exchange, r0 = atomic_cmpxchg((dst_reg + off), r0, src_reg) */
+
+#define BPF_ATOMIC_CMPXCHG(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_CMPXCHG })
+
/* Memory store, *(uint *) (dst_reg + off16) = imm32 */
#define BPF_ST_MEM(SIZE, DST, OFF, IMM) \
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 025e377e7229..82039a1176ac 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -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_XCHG (0xe0 | BPF_FETCH) /* atomic exchange */
+#define BPF_CMPXCHG (0xf0 | BPF_FETCH) /* atomic compare-and-write */
+#define BPF_FETCH 0x01 /* fetch previous value into src reg or r0*/
/* Register numbers */
enum {
--
2.29.2.454.gaff20da3a2-goog
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