* [PATCH bpf-next v4 1/2] bpf: Do not include stack ptr register in precision backtracking bookkeeping
@ 2025-05-22 5:02 Yonghong Song
2025-05-22 5:02 ` [PATCH bpf-next v4 2/2] selftests/bpf: Add tests with stack ptr register in conditional jmp Yonghong Song
2025-05-22 20:22 ` [PATCH bpf-next v4 1/2] bpf: Do not include stack ptr register in precision backtracking bookkeeping Andrii Nakryiko
0 siblings, 2 replies; 4+ messages in thread
From: Yonghong Song @ 2025-05-22 5:02 UTC (permalink / raw)
To: bpf
Cc: Alexei Starovoitov, Andrii Nakryiko, Daniel Borkmann, kernel-team,
Martin KaFai Lau
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 for 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 a stack pointer,
push_insn_history() will record such information, and later backtrack_insn()
will do bt_set_reg() properly for those register(s).
[1] https://lore.kernel.org/bpf/Z%2F8q3xzpU59CIYQE@ly-workstation/
Reported by: Yi Lai <yi1.lai@linux.intel.com>
Fixes: 407958a0e980 ("bpf: encapsulate precision backtracking bookkeeping")
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
include/linux/bpf_verifier.h | 12 +++++--
kernel/bpf/verifier.c | 68 ++++++++++++++++++++++++++++++++----
2 files changed, 70 insertions(+), 10 deletions(-)
Changelogs:
v3 -> v4:
- v3: https://lore.kernel.org/bpf/20250521170409.2772304-1-yonghong.song@linux.dev/
- Fix an issue in push_cond_jmp_history(). Previously, '!src_reg' was used to
check whether insn is 'dreg <op> imm' or not. But actually '!src_reg' is always
non-NULL. The new fix is using insn directly.
v2 -> v3:
- v2: https://lore.kernel.org/bpf/20250516161029.962760-1-yonghong.song@linux.dev/
- In v2, I put sreg_flag/dreg_flag into bpf_insn_hist_entry and the information
includes register numbers. This is not necessary as later insn in backtracking
can retrieve the register number. So the new change is remove sreg_flag/dreg_flag
from bpf_insn_hist_entry and add two bits in bpf_insn_hist_entry.flags to
record whether the registers (cond jump like <reg> op < reg>) are stack pointer
or not. Other changes depend on this data structure change.
v1 -> v2:
- v1: https://lore.kernel.org/bpf/20250511162758.281071-1-yonghong.song@linux.dev/
- 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 78c97e12ea4e..e73a910e4ece 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -357,6 +357,10 @@ 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_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 are used now. */
};
static_assert(INSN_F_FRAMENO_MASK + 1 >= MAX_CALL_FRAMES);
@@ -365,9 +369,11 @@ static_assert(INSN_F_SPI_MASK + 1 >= MAX_BPF_STACK / 8);
struct bpf_insn_hist_entry {
u32 idx;
/* insn idx can't be bigger than 1 million */
- u32 prev_idx : 22;
- /* special flags, e.g., whether insn is doing register stack spill/load */
- u32 flags : 10;
+ u32 prev_idx : 20;
+ /* special flags, e.g., whether insn is doing register stack spill/load,
+ * whether dst/src register is PTR_TO_STACK.
+ */
+ u32 flags : 12;
/* additional registers that need precision tracking when this
* jump is backtracked, vector of six 10-bit records
*/
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index d5807d2efc92..e295be7754cd 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3739,6 +3739,22 @@ static int check_reg_arg(struct bpf_verifier_env *env, u32 regno,
return __check_reg_arg(env, state->regs, regno, t);
}
+static int insn_reg_access_flags(bool dreg_stack_ptr, bool sreg_stack_ptr)
+{
+ return (dreg_stack_ptr ? INSN_F_DST_REG_STACK : 0) |
+ (sreg_stack_ptr ? INSN_F_SRC_REG_STACK : 0);
+}
+
+static bool insn_dreg_stack_ptr(int insn_flags)
+{
+ return !!(insn_flags & INSN_F_DST_REG_STACK);
+}
+
+static bool insn_sreg_stack_ptr(int insn_flags)
+{
+ return !!(insn_flags & INSN_F_SRC_REG_STACK);
+}
+
static int insn_stack_access_flags(int frameno, int spi)
{
return INSN_F_STACK_ACCESS | (spi << INSN_F_SPI_SHIFT) | frameno;
@@ -4402,6 +4418,8 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx, int subseq_idx,
*/
return 0;
} else if (BPF_SRC(insn->code) == BPF_X) {
+ bool dreg_precise, sreg_precise;
+
if (!bt_is_reg_set(bt, dreg) && !bt_is_reg_set(bt, sreg))
return 0;
/* dreg <cond> sreg
@@ -4410,8 +4428,16 @@ 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 = !insn_dreg_stack_ptr(hist->flags);
+ sreg_precise = !insn_sreg_stack_ptr(hist->flags);
+ 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
@@ -16397,6 +16423,29 @@ 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, struct bpf_reg_state *dst_reg,
+ struct bpf_reg_state *src_reg, u64 linked_regs)
+{
+ bool dreg_stack_ptr, sreg_stack_ptr;
+ int insn_flags;
+
+ if (BPF_SRC(insn->code) != BPF_X) {
+ if (linked_regs)
+ return push_insn_history(env, state, 0, linked_regs);
+ return 0;
+ }
+
+ dreg_stack_ptr = dst_reg->type == PTR_TO_STACK;
+ sreg_stack_ptr = src_reg->type == PTR_TO_STACK;
+
+ if (!dreg_stack_ptr && !sreg_stack_ptr && !linked_regs)
+ return 0;
+
+ insn_flags = insn_reg_access_flags(dreg_stack_ptr, sreg_stack_ptr);
+ return push_insn_history(env, state, insn_flags, linked_regs);
+}
+
static int check_cond_jmp_op(struct bpf_verifier_env *env,
struct bpf_insn *insn, int *insn_idx)
{
@@ -16500,6 +16549,9 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
!sanitize_speculative_path(env, insn, *insn_idx + 1,
*insn_idx))
return -EFAULT;
+ err = push_cond_jmp_history(env, this_branch, insn, dst_reg, src_reg, 0);
+ if (err)
+ return err;
if (env->log.level & BPF_LOG_LEVEL)
print_insn_state(env, this_branch, this_branch->curframe);
*insn_idx += insn->off;
@@ -16514,6 +16566,9 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
*insn_idx + insn->off + 1,
*insn_idx))
return -EFAULT;
+ err = push_cond_jmp_history(env, this_branch, insn, dst_reg, src_reg, 0);
+ if (err)
+ return err;
if (env->log.level & BPF_LOG_LEVEL)
print_insn_state(env, this_branch, this_branch->curframe);
return 0;
@@ -16528,11 +16583,10 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
collect_linked_regs(this_branch, src_reg->id, &linked_regs);
if (dst_reg->type == SCALAR_VALUE && dst_reg->id)
collect_linked_regs(this_branch, dst_reg->id, &linked_regs);
- if (linked_regs.cnt > 1) {
- err = push_insn_history(env, this_branch, 0, linked_regs_pack(&linked_regs));
- if (err)
- return err;
- }
+ err = push_cond_jmp_history(env, this_branch, insn, dst_reg, src_reg,
+ linked_regs.cnt > 1 ? linked_regs_pack(&linked_regs) : 0);
+ if (err)
+ return err;
other_branch = push_stack(env, *insn_idx + insn->off + 1, *insn_idx,
false);
--
2.47.1
^ permalink raw reply related [flat|nested] 4+ messages in thread
* [PATCH bpf-next v4 2/2] selftests/bpf: Add tests with stack ptr register in conditional jmp
2025-05-22 5:02 [PATCH bpf-next v4 1/2] bpf: Do not include stack ptr register in precision backtracking bookkeeping Yonghong Song
@ 2025-05-22 5:02 ` Yonghong Song
2025-05-22 20:22 ` [PATCH bpf-next v4 1/2] bpf: Do not include stack ptr register in precision backtracking bookkeeping Andrii Nakryiko
1 sibling, 0 replies; 4+ messages in thread
From: Yonghong Song @ 2025-05-22 5:02 UTC (permalink / raw)
To: bpf
Cc: Alexei Starovoitov, Andrii Nakryiko, Daniel Borkmann, kernel-team,
Martin KaFai Lau
Add two tests:
- one test has 'rX <op> r10' where rX is not r10, and
- another test has 'rX <op> rY' where rX and rY are not r10
but there is an early insn 'rX = r10'.
Without previous verifier change, both tests will fail.
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
.../selftests/bpf/progs/verifier_precision.c | 53 +++++++++++++++++++
1 file changed, 53 insertions(+)
diff --git a/tools/testing/selftests/bpf/progs/verifier_precision.c b/tools/testing/selftests/bpf/progs/verifier_precision.c
index 2dd0d15c2678..9fe5d255ee37 100644
--- a/tools/testing/selftests/bpf/progs/verifier_precision.c
+++ b/tools/testing/selftests/bpf/progs/verifier_precision.c
@@ -178,4 +178,57 @@ __naked int state_loop_first_last_equal(void)
);
}
+__used __naked static void __bpf_cond_op_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")
+__success __log_level(2)
+__msg("8: (bd) if r2 <= r10 goto pc+3")
+__msg("9: (35) if r1 >= 0xffe3fff8 goto pc+0")
+__msg("10: (b5) if r2 <= 0x8 goto pc+0")
+__msg("mark_precise: frame1: last_idx 10 first_idx 0 subseq_idx -1")
+__msg("mark_precise: frame1: regs=r2 stack= before 9: (35) if r1 >= 0xffe3fff8 goto pc+0")
+__msg("mark_precise: frame1: regs=r2 stack= before 8: (bd) if r2 <= r10 goto pc+3")
+__msg("mark_precise: frame1: regs=r2 stack= before 7: (05) goto pc+0")
+__naked void bpf_cond_op_r10(void)
+{
+ asm volatile (
+ "r3 = 0 ll;"
+ "call __bpf_cond_op_r10;"
+ "r0 = 0;"
+ "exit;"
+ ::: __clobber_all);
+}
+
+SEC("?raw_tp")
+__success __log_level(2)
+__msg("3: (bf) r3 = r10")
+__msg("4: (bd) if r3 <= r2 goto pc+1")
+__msg("5: (b5) if r2 <= 0x8 goto pc+2")
+__msg("mark_precise: frame0: last_idx 5 first_idx 0 subseq_idx -1")
+__msg("mark_precise: frame0: regs=r2 stack= before 4: (bd) if r3 <= r2 goto pc+1")
+__msg("mark_precise: frame0: regs=r2 stack= before 3: (bf) r3 = r10")
+__naked void bpf_cond_op_not_r10(void)
+{
+ asm volatile (
+ "r0 = 0;"
+ "r2 = 2314885393468386424 ll;"
+ "r3 = r10;"
+ "if r3 <= r2 goto +1;"
+ "if r2 <= 8 goto +2;"
+ "r0 = 2 ll;"
+ "exit;"
+ ::: __clobber_all);
+}
+
char _license[] SEC("license") = "GPL";
--
2.47.1
^ permalink raw reply related [flat|nested] 4+ messages in thread
* Re: [PATCH bpf-next v4 1/2] bpf: Do not include stack ptr register in precision backtracking bookkeeping
2025-05-22 5:02 [PATCH bpf-next v4 1/2] bpf: Do not include stack ptr register in precision backtracking bookkeeping Yonghong Song
2025-05-22 5:02 ` [PATCH bpf-next v4 2/2] selftests/bpf: Add tests with stack ptr register in conditional jmp Yonghong Song
@ 2025-05-22 20:22 ` Andrii Nakryiko
2025-05-24 0:27 ` Yonghong Song
1 sibling, 1 reply; 4+ messages in thread
From: Andrii Nakryiko @ 2025-05-22 20:22 UTC (permalink / raw)
To: Yonghong Song
Cc: bpf, Alexei Starovoitov, Andrii Nakryiko, Daniel Borkmann,
kernel-team, Martin KaFai Lau
On Wed, May 21, 2025 at 10:02 PM Yonghong Song <yonghong.song@linux.dev> 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 for 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 a stack pointer,
> push_insn_history() will record such information, and later backtrack_insn()
> will do bt_set_reg() properly for those register(s).
>
> [1] https://lore.kernel.org/bpf/Z%2F8q3xzpU59CIYQE@ly-workstation/
>
> Reported by: Yi Lai <yi1.lai@linux.intel.com>
> Fixes: 407958a0e980 ("bpf: encapsulate precision backtracking bookkeeping")
> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
> ---
> include/linux/bpf_verifier.h | 12 +++++--
> kernel/bpf/verifier.c | 68 ++++++++++++++++++++++++++++++++----
> 2 files changed, 70 insertions(+), 10 deletions(-)
>
> Changelogs:
> v3 -> v4:
> - v3: https://lore.kernel.org/bpf/20250521170409.2772304-1-yonghong.song@linux.dev/
> - Fix an issue in push_cond_jmp_history(). Previously, '!src_reg' was used to
> check whether insn is 'dreg <op> imm' or not. But actually '!src_reg' is always
> non-NULL. The new fix is using insn directly.
> v2 -> v3:
> - v2: https://lore.kernel.org/bpf/20250516161029.962760-1-yonghong.song@linux.dev/
> - In v2, I put sreg_flag/dreg_flag into bpf_insn_hist_entry and the information
> includes register numbers. This is not necessary as later insn in backtracking
> can retrieve the register number. So the new change is remove sreg_flag/dreg_flag
> from bpf_insn_hist_entry and add two bits in bpf_insn_hist_entry.flags to
> record whether the registers (cond jump like <reg> op < reg>) are stack pointer
> or not. Other changes depend on this data structure change.
> v1 -> v2:
> - v1: https://lore.kernel.org/bpf/20250511162758.281071-1-yonghong.song@linux.dev/
> - 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 78c97e12ea4e..e73a910e4ece 100644
> --- a/include/linux/bpf_verifier.h
> +++ b/include/linux/bpf_verifier.h
> @@ -357,6 +357,10 @@ enum {
> INSN_F_SPI_SHIFT = 3, /* shifted 3 bits to the left */
>
> INSN_F_STACK_ACCESS = BIT(9), /* we need 10 bits total */
drop this comment, it's now even more misleading than it was before :)
> +
> + 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 are used now. */
> };
>
> static_assert(INSN_F_FRAMENO_MASK + 1 >= MAX_CALL_FRAMES);
> @@ -365,9 +369,11 @@ static_assert(INSN_F_SPI_MASK + 1 >= MAX_BPF_STACK / 8);
> struct bpf_insn_hist_entry {
> u32 idx;
> /* insn idx can't be bigger than 1 million */
> - u32 prev_idx : 22;
> - /* special flags, e.g., whether insn is doing register stack spill/load */
> - u32 flags : 10;
> + u32 prev_idx : 20;
> + /* special flags, e.g., whether insn is doing register stack spill/load,
> + * whether dst/src register is PTR_TO_STACK.
we probably don't need to duplicate the list of meanings of those
flags, just mention that these are INSN_F_xxx flags from the above?
> + */
> + u32 flags : 12;
> /* additional registers that need precision tracking when this
> * jump is backtracked, vector of six 10-bit records
> */
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index d5807d2efc92..e295be7754cd 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -3739,6 +3739,22 @@ static int check_reg_arg(struct bpf_verifier_env *env, u32 regno,
> return __check_reg_arg(env, state->regs, regno, t);
> }
>
> +static int insn_reg_access_flags(bool dreg_stack_ptr, bool sreg_stack_ptr)
"reg_access" isn't really precise, no? I actually don't see much
benefit for this and insn_[sd]reg_stack_ptr() helpers, they literally
set/get singular bits. It's a bit different for STACK_ACCESS one where
we have bit + integer with mask. Let's keep it simple?
> +{
> + return (dreg_stack_ptr ? INSN_F_DST_REG_STACK : 0) |
> + (sreg_stack_ptr ? INSN_F_SRC_REG_STACK : 0);
> +}
> +
> +static bool insn_dreg_stack_ptr(int insn_flags)
> +{
> + return !!(insn_flags & INSN_F_DST_REG_STACK);
> +}
> +
> +static bool insn_sreg_stack_ptr(int insn_flags)
> +{
> + return !!(insn_flags & INSN_F_SRC_REG_STACK);
> +}
> +
> static int insn_stack_access_flags(int frameno, int spi)
> {
> return INSN_F_STACK_ACCESS | (spi << INSN_F_SPI_SHIFT) | frameno;
> @@ -4402,6 +4418,8 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx, int subseq_idx,
> */
> return 0;
> } else if (BPF_SRC(insn->code) == BPF_X) {
> + bool dreg_precise, sreg_precise;
> +
> if (!bt_is_reg_set(bt, dreg) && !bt_is_reg_set(bt, sreg))
> return 0;
> /* dreg <cond> sreg
> @@ -4410,8 +4428,16 @@ 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;
hm... I'd assume that we need hist only if there is some special
condition (PTR_TO_STACK registers), and otherwise we should assume
that registers are not special.
> + dreg_precise = !insn_dreg_stack_ptr(hist->flags);
> + sreg_precise = !insn_sreg_stack_ptr(hist->flags);
> + if (!dreg_precise && !sreg_precise)
> + return 0;
> + if (dreg_precise)
> + bt_set_reg(bt, dreg);
> + if (sreg_precise)
> + bt_set_reg(bt, sreg);
and so with the above we'll have
if (!hist || !(hist->flags & INSN_F_SRC_REG_STACK))
bt_set_reg(bt, sreg);
if (!hist || !(hist->flags & INSN_F_DST_REG_STACK))
bt_set_reg(bt, dreg);
That inversion of conditions is a bit unfortunate, but it's pretty
contained and small, so probably just fine
> } else if (BPF_SRC(insn->code) == BPF_K) {
> /* dreg <cond> K
> * Only dreg still needs precision before
> @@ -16397,6 +16423,29 @@ 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, struct bpf_reg_state *dst_reg,
> + struct bpf_reg_state *src_reg, u64 linked_regs)
> +{
> + bool dreg_stack_ptr, sreg_stack_ptr;
> + int insn_flags;
> +
> + if (BPF_SRC(insn->code) != BPF_X) {
> + if (linked_regs)
> + return push_insn_history(env, state, 0, linked_regs);
> + return 0;
> + }
> +
> + dreg_stack_ptr = dst_reg->type == PTR_TO_STACK;
> + sreg_stack_ptr = src_reg->type == PTR_TO_STACK;
> +
> + if (!dreg_stack_ptr && !sreg_stack_ptr && !linked_regs)
> + return 0;
> +
> + insn_flags = insn_reg_access_flags(dreg_stack_ptr, sreg_stack_ptr);
> + return push_insn_history(env, state, insn_flags, linked_regs);
> +}
> +
> static int check_cond_jmp_op(struct bpf_verifier_env *env,
> struct bpf_insn *insn, int *insn_idx)
> {
> @@ -16500,6 +16549,9 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
> !sanitize_speculative_path(env, insn, *insn_idx + 1,
> *insn_idx))
> return -EFAULT;
> + err = push_cond_jmp_history(env, this_branch, insn, dst_reg, src_reg, 0);
I think we are "overabstracting" here with additional helper. This
helper has to redo BPF_X special handling and stuff like that, but for
no good reason. Just do similar approach we have in
check_stack_read_fixed_off:
static int check_cond_jmp_op(...)
{
int insn_flags = 0;
...
if (BPF_SRC(insn->code) == BPF_X) {
if (insn->src_reg is PTR_TO_STACK)
insn_flags |= INSN_F_SRC_REG_STACK;
if (insn->dst_reg is PTR_TO_STACK)
insn_flags |= INSN_F_DST_REG_STACK;
} else {
if (insn->src_reg is PTR_TO_STACK)
insn_flags |= INSN_F_SRC_REG_STACK;
}
...
if (insn_flags) {
err = push_insn_history(env, ..., insn_flags, 0);
if (err)
return err;
}
if (linked_regs.cnt > 1) {
err = push_insn_history(env, ..., 0, <linked-regs-stuff>);
...
}}
Note, two push_insn_history() should be fine, check env->cur_hist_ent
handling inside it.
All these tiny small and one-time helpers are added cognitive load, so
unless they are really helpful let's avoid them. These flags are
simple enough to not need any of that, IMO.
pw-bot: cr
> + if (err)
> + return err;
> if (env->log.level & BPF_LOG_LEVEL)
> print_insn_state(env, this_branch, this_branch->curframe);
> *insn_idx += insn->off;
> @@ -16514,6 +16566,9 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
> *insn_idx + insn->off + 1,
> *insn_idx))
> return -EFAULT;
> + err = push_cond_jmp_history(env, this_branch, insn, dst_reg, src_reg, 0);
> + if (err)
> + return err;
> if (env->log.level & BPF_LOG_LEVEL)
> print_insn_state(env, this_branch, this_branch->curframe);
> return 0;
> @@ -16528,11 +16583,10 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
> collect_linked_regs(this_branch, src_reg->id, &linked_regs);
> if (dst_reg->type == SCALAR_VALUE && dst_reg->id)
> collect_linked_regs(this_branch, dst_reg->id, &linked_regs);
> - if (linked_regs.cnt > 1) {
> - err = push_insn_history(env, this_branch, 0, linked_regs_pack(&linked_regs));
> - if (err)
> - return err;
> - }
> + err = push_cond_jmp_history(env, this_branch, insn, dst_reg, src_reg,
> + linked_regs.cnt > 1 ? linked_regs_pack(&linked_regs) : 0);
> + if (err)
> + return err;
>
> other_branch = push_stack(env, *insn_idx + insn->off + 1, *insn_idx,
> false);
> --
> 2.47.1
>
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH bpf-next v4 1/2] bpf: Do not include stack ptr register in precision backtracking bookkeeping
2025-05-22 20:22 ` [PATCH bpf-next v4 1/2] bpf: Do not include stack ptr register in precision backtracking bookkeeping Andrii Nakryiko
@ 2025-05-24 0:27 ` Yonghong Song
0 siblings, 0 replies; 4+ messages in thread
From: Yonghong Song @ 2025-05-24 0:27 UTC (permalink / raw)
To: Andrii Nakryiko
Cc: bpf, Alexei Starovoitov, Andrii Nakryiko, Daniel Borkmann,
kernel-team, Martin KaFai Lau
On 5/22/25 1:22 PM, Andrii Nakryiko wrote:
> On Wed, May 21, 2025 at 10:02 PM Yonghong Song <yonghong.song@linux.dev> 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 for 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 a stack pointer,
>> push_insn_history() will record such information, and later backtrack_insn()
>> will do bt_set_reg() properly for those register(s).
>>
>> [1] https://lore.kernel.org/bpf/Z%2F8q3xzpU59CIYQE@ly-workstation/
>>
>> Reported by: Yi Lai <yi1.lai@linux.intel.com>
>> Fixes: 407958a0e980 ("bpf: encapsulate precision backtracking bookkeeping")
>> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
>> ---
>> include/linux/bpf_verifier.h | 12 +++++--
>> kernel/bpf/verifier.c | 68 ++++++++++++++++++++++++++++++++----
>> 2 files changed, 70 insertions(+), 10 deletions(-)
>>
>> Changelogs:
>> v3 -> v4:
>> - v3: https://lore.kernel.org/bpf/20250521170409.2772304-1-yonghong.song@linux.dev/
>> - Fix an issue in push_cond_jmp_history(). Previously, '!src_reg' was used to
>> check whether insn is 'dreg <op> imm' or not. But actually '!src_reg' is always
>> non-NULL. The new fix is using insn directly.
>> v2 -> v3:
>> - v2: https://lore.kernel.org/bpf/20250516161029.962760-1-yonghong.song@linux.dev/
>> - In v2, I put sreg_flag/dreg_flag into bpf_insn_hist_entry and the information
>> includes register numbers. This is not necessary as later insn in backtracking
>> can retrieve the register number. So the new change is remove sreg_flag/dreg_flag
>> from bpf_insn_hist_entry and add two bits in bpf_insn_hist_entry.flags to
>> record whether the registers (cond jump like <reg> op < reg>) are stack pointer
>> or not. Other changes depend on this data structure change.
>> v1 -> v2:
>> - v1: https://lore.kernel.org/bpf/20250511162758.281071-1-yonghong.song@linux.dev/
>> - 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 78c97e12ea4e..e73a910e4ece 100644
>> --- a/include/linux/bpf_verifier.h
>> +++ b/include/linux/bpf_verifier.h
>> @@ -357,6 +357,10 @@ enum {
>> INSN_F_SPI_SHIFT = 3, /* shifted 3 bits to the left */
>>
>> INSN_F_STACK_ACCESS = BIT(9), /* we need 10 bits total */
> drop this comment, it's now even more misleading than it was before :)
Good idea. Will do.
>
>> +
>> + 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 are used now. */
>> };
>>
>> static_assert(INSN_F_FRAMENO_MASK + 1 >= MAX_CALL_FRAMES);
>> @@ -365,9 +369,11 @@ static_assert(INSN_F_SPI_MASK + 1 >= MAX_BPF_STACK / 8);
>> struct bpf_insn_hist_entry {
>> u32 idx;
>> /* insn idx can't be bigger than 1 million */
>> - u32 prev_idx : 22;
>> - /* special flags, e.g., whether insn is doing register stack spill/load */
>> - u32 flags : 10;
>> + u32 prev_idx : 20;
>> + /* special flags, e.g., whether insn is doing register stack spill/load,
>> + * whether dst/src register is PTR_TO_STACK.
> we probably don't need to duplicate the list of meanings of those
> flags, just mention that these are INSN_F_xxx flags from the above?
Make sense. There is no need to change original comments.
>
>> + */
>> + u32 flags : 12;
>> /* additional registers that need precision tracking when this
>> * jump is backtracked, vector of six 10-bit records
>> */
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index d5807d2efc92..e295be7754cd 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -3739,6 +3739,22 @@ static int check_reg_arg(struct bpf_verifier_env *env, u32 regno,
>> return __check_reg_arg(env, state->regs, regno, t);
>> }
>>
>> +static int insn_reg_access_flags(bool dreg_stack_ptr, bool sreg_stack_ptr)
> "reg_access" isn't really precise, no? I actually don't see much
> benefit for this and insn_[sd]reg_stack_ptr() helpers, they literally
> set/get singular bits. It's a bit different for STACK_ACCESS one where
> we have bit + integer with mask. Let's keep it simple?
Originally I tried to have similar helpers for stack_access_flags etc.
But indeed register encoding is similar we can directly do encoding
in the use place.
>
>
>> +{
>> + return (dreg_stack_ptr ? INSN_F_DST_REG_STACK : 0) |
>> + (sreg_stack_ptr ? INSN_F_SRC_REG_STACK : 0);
>> +}
>> +
>> +static bool insn_dreg_stack_ptr(int insn_flags)
>> +{
>> + return !!(insn_flags & INSN_F_DST_REG_STACK);
>> +}
>> +
>> +static bool insn_sreg_stack_ptr(int insn_flags)
>> +{
>> + return !!(insn_flags & INSN_F_SRC_REG_STACK);
>> +}
>> +
>> static int insn_stack_access_flags(int frameno, int spi)
>> {
>> return INSN_F_STACK_ACCESS | (spi << INSN_F_SPI_SHIFT) | frameno;
>> @@ -4402,6 +4418,8 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx, int subseq_idx,
>> */
>> return 0;
>> } else if (BPF_SRC(insn->code) == BPF_X) {
>> + bool dreg_precise, sreg_precise;
>> +
>> if (!bt_is_reg_set(bt, dreg) && !bt_is_reg_set(bt, sreg))
>> return 0;
>> /* dreg <cond> sreg
>> @@ -4410,8 +4428,16 @@ 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;
> hm... I'd assume that we need hist only if there is some special
> condition (PTR_TO_STACK registers), and otherwise we should assume
> that registers are not special.
For rX <op> rY operation, the hist is NULL with the following condition:
rX->type != PTR_TO_STACK && rY->type != PTR_STACK && !linked_regs
>
>> + dreg_precise = !insn_dreg_stack_ptr(hist->flags);
>> + sreg_precise = !insn_sreg_stack_ptr(hist->flags);
>> + if (!dreg_precise && !sreg_precise)
>> + return 0;
>> + if (dreg_precise)
>> + bt_set_reg(bt, dreg);
>> + if (sreg_precise)
>> + bt_set_reg(bt, sreg);
> and so with the above we'll have
>
>
> if (!hist || !(hist->flags & INSN_F_SRC_REG_STACK))
> bt_set_reg(bt, sreg);
> if (!hist || !(hist->flags & INSN_F_DST_REG_STACK))
> bt_set_reg(bt, dreg);
So indeed this is correct. I somehow missed it.
>
> That inversion of conditions is a bit unfortunate, but it's pretty
> contained and small, so probably just fine
>
>
>> } else if (BPF_SRC(insn->code) == BPF_K) {
>> /* dreg <cond> K
>> * Only dreg still needs precision before
>> @@ -16397,6 +16423,29 @@ 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, struct bpf_reg_state *dst_reg,
>> + struct bpf_reg_state *src_reg, u64 linked_regs)
>> +{
>> + bool dreg_stack_ptr, sreg_stack_ptr;
>> + int insn_flags;
>> +
>> + if (BPF_SRC(insn->code) != BPF_X) {
>> + if (linked_regs)
>> + return push_insn_history(env, state, 0, linked_regs);
>> + return 0;
>> + }
>> +
>> + dreg_stack_ptr = dst_reg->type == PTR_TO_STACK;
>> + sreg_stack_ptr = src_reg->type == PTR_TO_STACK;
>> +
>> + if (!dreg_stack_ptr && !sreg_stack_ptr && !linked_regs)
>> + return 0;
>> +
>> + insn_flags = insn_reg_access_flags(dreg_stack_ptr, sreg_stack_ptr);
>> + return push_insn_history(env, state, insn_flags, linked_regs);
>> +}
>> +
>> static int check_cond_jmp_op(struct bpf_verifier_env *env,
>> struct bpf_insn *insn, int *insn_idx)
>> {
>> @@ -16500,6 +16549,9 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
>> !sanitize_speculative_path(env, insn, *insn_idx + 1,
>> *insn_idx))
>> return -EFAULT;
>> + err = push_cond_jmp_history(env, this_branch, insn, dst_reg, src_reg, 0);
> I think we are "overabstracting" here with additional helper. This
> helper has to redo BPF_X special handling and stuff like that, but for
> no good reason. Just do similar approach we have in
> check_stack_read_fixed_off:
>
> static int check_cond_jmp_op(...)
> {
> int insn_flags = 0;
> ...
>
>
> if (BPF_SRC(insn->code) == BPF_X) {
> if (insn->src_reg is PTR_TO_STACK)
> insn_flags |= INSN_F_SRC_REG_STACK;
> if (insn->dst_reg is PTR_TO_STACK)
> insn_flags |= INSN_F_DST_REG_STACK;
> } else {
> if (insn->src_reg is PTR_TO_STACK)
> insn_flags |= INSN_F_SRC_REG_STACK;
> }
>
> ...
> if (insn_flags) {
> err = push_insn_history(env, ..., insn_flags, 0);
> if (err)
> return err;
> }
> if (linked_regs.cnt > 1) {
> err = push_insn_history(env, ..., 0, <linked-regs-stuff>);
> ...
> }}
>
> Note, two push_insn_history() should be fine, check env->cur_hist_ent
> handling inside it.
Good point with two push_insn_history(). The code insn_flags handling
is similar to check_stack_{read,write}_fixed_off. Will follow the
same implementation style as the above.
>
> All these tiny small and one-time helpers are added cognitive load, so
> unless they are really helpful let's avoid them. These flags are
> simple enough to not need any of that, IMO.
Ack.
>
>
> pw-bot: cr
>
>> + if (err)
>> + return err;
>> if (env->log.level & BPF_LOG_LEVEL)
>> print_insn_state(env, this_branch, this_branch->curframe);
>> *insn_idx += insn->off;
>> @@ -16514,6 +16566,9 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
>> *insn_idx + insn->off + 1,
>> *insn_idx))
>> return -EFAULT;
>> + err = push_cond_jmp_history(env, this_branch, insn, dst_reg, src_reg, 0);
>> + if (err)
>> + return err;
>> if (env->log.level & BPF_LOG_LEVEL)
>> print_insn_state(env, this_branch, this_branch->curframe);
>> return 0;
>> @@ -16528,11 +16583,10 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
>> collect_linked_regs(this_branch, src_reg->id, &linked_regs);
>> if (dst_reg->type == SCALAR_VALUE && dst_reg->id)
>> collect_linked_regs(this_branch, dst_reg->id, &linked_regs);
>> - if (linked_regs.cnt > 1) {
>> - err = push_insn_history(env, this_branch, 0, linked_regs_pack(&linked_regs));
>> - if (err)
>> - return err;
>> - }
>> + err = push_cond_jmp_history(env, this_branch, insn, dst_reg, src_reg,
>> + linked_regs.cnt > 1 ? linked_regs_pack(&linked_regs) : 0);
>> + if (err)
>> + return err;
>>
>> other_branch = push_stack(env, *insn_idx + insn->off + 1, *insn_idx,
>> false);
>> --
>> 2.47.1
>>
^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2025-05-24 0:27 UTC | newest]
Thread overview: 4+ messages (download: mbox.gz follow: Atom feed
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2025-05-22 5:02 [PATCH bpf-next v4 1/2] bpf: Do not include stack ptr register in precision backtracking bookkeeping Yonghong Song
2025-05-22 5:02 ` [PATCH bpf-next v4 2/2] selftests/bpf: Add tests with stack ptr register in conditional jmp Yonghong Song
2025-05-22 20:22 ` [PATCH bpf-next v4 1/2] bpf: Do not include stack ptr register in precision backtracking bookkeeping Andrii Nakryiko
2025-05-24 0:27 ` Yonghong Song
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