On 5/20/25 7:16 AM, Andrii Nakryiko wrote:
On Fri, May 16, 2025 at 9:10 AM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
Yi Lai reported an issue ([1]) where the following warning appears
in kernel dmesg:
[ 60.643604] verifier backtracking bug
[ 60.643635] WARNING: CPU: 10 PID: 2315 at kernel/bpf/verifier.c:4302 __mark_chain_precision+0x3a6c/0x3e10
[ 60.648428] Modules linked in: bpf_testmod(OE)
[ 60.650471] CPU: 10 UID: 0 PID: 2315 Comm: test_progs Tainted: G OE 6.15.0-rc4-gef11287f8289-dirty #327 PREEMPT(full)
[ 60.654385] Tainted: [O]=OOT_MODULE, [E]=UNSIGNED_MODULE
[ 60.656682] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
[ 60.660475] RIP: 0010:__mark_chain_precision+0x3a6c/0x3e10
[ 60.662814] Code: 5a 30 84 89 ea e8 c4 d9 01 00 80 3d 3e 7d d8 04 00 0f 85 60 fa ff ff c6 05 31 7d d8 04
01 48 c7 c7 00 58 30 84 e8 c4 06 a5 ff <0f> 0b e9 46 fa ff ff 48 ...
[ 60.668720] RSP: 0018:ffff888116cc7298 EFLAGS: 00010246
[ 60.671075] RAX: 54d70e82dfd31900 RBX: ffff888115b65e20 RCX: 0000000000000000
[ 60.673659] RDX: 0000000000000001 RSI: 0000000000000004 RDI: 00000000ffffffff
[ 60.676241] RBP: 0000000000000400 R08: ffff8881f6f23bd3 R09: 1ffff1103ede477a
[ 60.678787] R10: dffffc0000000000 R11: ffffed103ede477b R12: ffff888115b60ae8
[ 60.681420] R13: 1ffff11022b6cbc4 R14: 00000000fffffff2 R15: 0000000000000001
[ 60.684030] FS: 00007fc2aedd80c0(0000) GS:ffff88826fa8a000(0000) knlGS:0000000000000000
[ 60.686837] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 60.689027] CR2: 000056325369e000 CR3: 000000011088b002 CR4: 0000000000370ef0
[ 60.691623] Call Trace:
[ 60.692821] <TASK>
[ 60.693960] ? __pfx_verbose+0x10/0x10
[ 60.695656] ? __pfx_disasm_kfunc_name+0x10/0x10
[ 60.697495] check_cond_jmp_op+0x16f7/0x39b0
[ 60.699237] do_check+0x58fa/0xab10
...
Further analysis shows the warning is at line 4302 as below:
4294 /* static subprog call instruction, which
4295 * means that we are exiting current subprog,
4296 * so only r1-r5 could be still requested as
4297 * precise, r0 and r6-r10 or any stack slot in
4298 * the current frame should be zero by now
4299 */
4300 if (bt_reg_mask(bt) & ~BPF_REGMASK_ARGS) {
4301 verbose(env, "BUG regs %x\n", bt_reg_mask(bt));
4302 WARN_ONCE(1, "verifier backtracking bug");
4303 return -EFAULT;
4304 }
With the below test (also in the next patch):
__used __naked static void __bpf_jmp_r10(void)
{
asm volatile (
"r2 = 2314885393468386424 ll;"
"goto +0;"
"if r2 <= r10 goto +3;"
"if r1 >= -1835016 goto +0;"
"if r2 <= 8 goto +0;"
"if r3 <= 0 goto +0;"
"exit;"
::: __clobber_all);
}
SEC("?raw_tp")
__naked void bpf_jmp_r10(void)
{
asm volatile (
"r3 = 0 ll;"
"call __bpf_jmp_r10;"
"r0 = 0;"
"exit;"
::: __clobber_all);
}
The following is the verifier failure log:
0: (18) r3 = 0x0 ; R3_w=0
2: (85) call pc+2
caller:
R10=fp0
callee:
frame1: R1=ctx() R3_w=0 R10=fp0
5: frame1: R1=ctx() R3_w=0 R10=fp0
; asm volatile (" \ @ verifier_precision.c:184
5: (18) r2 = 0x20202000256c6c78 ; frame1: R2_w=0x20202000256c6c78
7: (05) goto pc+0
8: (bd) if r2 <= r10 goto pc+3 ; frame1: R2_w=0x20202000256c6c78 R10=fp0
9: (35) if r1 >= 0xffe3fff8 goto pc+0 ; frame1: R1=ctx()
10: (b5) if r2 <= 0x8 goto pc+0
mark_precise: frame1: last_idx 10 first_idx 0 subseq_idx -1
mark_precise: frame1: regs=r2 stack= before 9: (35) if r1 >= 0xffe3fff8 goto pc+0
mark_precise: frame1: regs=r2 stack= before 8: (bd) if r2 <= r10 goto pc+3
mark_precise: frame1: regs=r2,r10 stack= before 7: (05) goto pc+0
mark_precise: frame1: regs=r2,r10 stack= before 5: (18) r2 = 0x20202000256c6c78
mark_precise: frame1: regs=r10 stack= before 2: (85) call pc+2
BUG regs 400
The main failure reason is due to r10 in precision backtracking bookkeeping.
Actually r10 is always precise and there is no need to add it the precision
backtracking bookkeeping.
One way to fix the issue is to prevent bt_set_reg() if any src/dst reg is
r10. Andrii suggested to go with push_insn_history() approach to avoid
explicitly checking r10 in backtrack_insn().
This patch added push_insn_history() support for cond_jmp like 'rX <op> rY'
operations. In check_cond_jmp_op(), if any of rX or rY is r10, push_insn_history()
will not record that register, and later backtrack_insn() will not do
bt_set_reg() for those registers.
[1] https://lore.kernel.org/bpf/Z%2F8q3xzpU59CIYQE@ly-workstation/
Reported by: Yi Lai <yi1.lai@xxxxxxxxxxxxxxx>
Fixes: 407958a0e980 ("bpf: encapsulate precision backtracking bookkeeping")
Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx>
---
include/linux/bpf_verifier.h | 7 ++++
kernel/bpf/verifier.c | 69 +++++++++++++++++++++++++++++-------
2 files changed, 64 insertions(+), 12 deletions(-)
Changelogs:
v1 -> v2:
- v1: https://lore.kernel.org/bpf/20250511162758.281071-1-yonghong.song@xxxxxxxxx/
- In v1, we check r10 register explicitly in backtrack_insn() to decide
whether we should do bt_set_reg() or not. Andrii suggested to do
push_insn_history() instead. Whether a particular register (r10 in this case)
should be available for backtracking or not is in check_cond_jmp_op(),
and such information is pushed with push_insn_history(). Later in backtrack_insn(),
such info is retrieved to decide whether precision marking should be
done or not. This apporach can avoid explicit checking for r10 in backtrack_insn().
diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index cedd66867ecf..9d3fdabeeaf4 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -357,6 +357,8 @@ enum {
INSN_F_SPI_SHIFT = 3, /* shifted 3 bits to the left */
INSN_F_STACK_ACCESS = BIT(9), /* we need 10 bits total */
+
+ INSN_F_REG_ACCESS = BIT(4), /* we need 5 bits total */
hm... you are actually clashing with INSN_F_SPI_MASK here, bits 3
through 8 are used to record stack slot index.
The reason I did this is that INSN_F_REG_ACCESS is used for
later struct bpf_insn_hist_entry sreg_flag and dreg_flag.
But as you mentioned later, there is no need to save register
numbers in sreg_flag/dreg_flag as insn already has it ...
I think we should go with
INSN_F_DST_REG_STACK = BIT(10), /* dst_reg is PTR_TO_STACK */
INSN_F_SRC_REG_STACK = BIT(11), /* src_reg is PTR_TO_STACK */
/* total 12 bits total can used now */
also note that all this stuff needs to fit into
bpf_insn_hist_entry.flags, which is currently set to be 10 bits, and
so now we need two extra bits.
Luckily, prev_idx: 22 doesn't really need 22 bits, so we can steal two
bits there and still be able to express 1 million instructions
indices:
u32 prev_idx : 20;
u32 flags : 12;
The above works since we do not need to save register numbers
in bpf_insn_hist_entry. I will rework along this line.
pw-bot: cr
};
static_assert(INSN_F_FRAMENO_MASK + 1 >= MAX_CALL_FRAMES);
@@ -372,6 +374,11 @@ struct bpf_insn_hist_entry {
* jump is backtracked, vector of six 10-bit records
*/
u64 linked_regs;
+ /* special flag, e.g., whether reg is used for non-load/store insns
+ * during precision backtracking.
+ */
+ u8 sreg_flag;
+ u8 dreg_flag;
this is not necessary, src_reg and dst_reg number itself is coming
from the bpf_insn itself?
You are correct. sreg_flag/dreg_flag is not needed.
INSN_F_{DST,SRC}_REG_STACK should be enough.
[...]
@@ -4414,8 +4425,16 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx, int subseq_idx,
* before it would be equally necessary to
* propagate it to dreg.
*/
- bt_set_reg(bt, dreg);
- bt_set_reg(bt, sreg);
+ if (!hist)
+ return 0;
+ dreg_precise = hist->dreg_flag == insn_reg_with_access_flag(dreg);
+ sreg_precise = hist->sreg_flag == insn_reg_with_access_flag(sreg);
As I mentioned above, we don't need to store dst_reg and src_reg
numbers themselves, we are getting them from bpf_insn. We just need
those two flags, INSN_F_DST_REG_STACK and INSN_F_SRC_REG_STACK, to
know which registers were PTR_TO_STACK at the time when we validated
that instruction
Ack.
+ if (!dreg_precise && !sreg_precise)
+ return 0;
+ if (dreg_precise)
+ bt_set_reg(bt, dreg);
+ if (sreg_precise)
+ bt_set_reg(bt, sreg);
} else if (BPF_SRC(insn->code) == BPF_K) {
/* dreg <cond> K
* Only dreg still needs precision before
@@ -5115,7 +5134,7 @@ static int check_stack_write_fixed_off(struct bpf_verifier_env *env,
}
if (insn_flags)
- return push_insn_history(env, env->cur_state, insn_flags, 0);
+ return push_insn_history(env, env->cur_state, insn_flags, 0, 0, 0);
return 0;
}
@@ -5422,7 +5441,7 @@ static int check_stack_read_fixed_off(struct bpf_verifier_env *env,
insn_flags = 0; /* we are not restoring spilled register */
}
if (insn_flags)
- return push_insn_history(env, env->cur_state, insn_flags, 0);
+ return push_insn_history(env, env->cur_state, insn_flags, 0, 0, 0);
return 0;
}
@@ -16414,6 +16433,27 @@ static void sync_linked_regs(struct bpf_verifier_state *vstate, struct bpf_reg_s
}
}
+static int push_cond_jmp_history(struct bpf_verifier_env *env, struct bpf_verifier_state *state,
+ struct bpf_insn *insn, u64 linked_regs)
+{
+ int err;
+
+ if ((BPF_SRC(insn->code) != BPF_X ||
+ (insn->src_reg == BPF_REG_FP && insn->dst_reg == BPF_REG_FP)) &&
we shouldn't be checking for BPF_REG_FP here either. Look up
bpf_reg_state by insn->src_reg, and check that it's PTR_TO_STACK
Correct. Will check reg->type == PTR_TO_STACK instead.
+ !linked_regs)
+ return 0;
+
+ err = push_insn_history(env, state, 0, linked_regs,
+ BPF_SRC(insn->code) == BPF_X && insn->src_reg != BPF_REG_FP
+ ? insn_reg_with_access_flag(insn->src_reg)
+ : 0,
+ BPF_SRC(insn->code) == BPF_X && insn->dst_reg != BPF_REG_FP
+ ? insn_reg_with_access_flag(insn->dst_reg)
+ : 0);
+
+ return err;
+}
+
[...]
