linux-stable.git/kernel/bpf/backtrack.c, branch v7.3-rc2

bpf: backtracking shouldn't clear outer frame R1-R5 for callbacks

2026-09-02T18:13:21+00:00

When processing calls to bpf_loop() verifier marks R1 (and R4) as
precise. R1 tracks loop iterations number and because of the
'callback_depth < R1' mechanics in check_helper_call() must be marked
precise. However, precision propagation for R1 was broken,
when bpf_loop() call was verified on a second iteration.

Consider the following verification trace:
- main: bpf_loop(nr_loops, callback ...)
- callback: BPF_EXIT
- main: bpf_loop(nr_loops, callback ...)
- ...

While the first visit of the call to bpf_loop() propagated R1
precision as expected, the second call to mark_chain_precision() in
the check_helper_call() set R1, but it was immediately reset when
backtrack_insn() processed preceding BPF_EXIT in the loop deleted in
this patch.

Because of that, the second visit of the call to bpf_loop() injected
checkpoint with R1 not marked as precise. Which could trick the
verifier into accepting unsafe programs. See the next patch for an
example of such program.

Commit is structured in a way to minimize conflicts when
'bpf' would be eventually merged with 'bpf-next'.

Fixes: ab5cfac139ab ("bpf: verify callbacks as if they are called unknown number of times")
Reported-by: Nicholas Carlini 
Suggested-by: Nicholas Carlini 
Signed-off-by: Eduard Zingerman 
Link: https://lore.kernel.org/r/20260831-bug-015-backtrack-cb-args-precise-v1-1-68a8e2a821e0@gmail.com
Signed-off-by: Alexei Starovoitov

bpf: backtrack_insn(): Handle ld_{abs,ind} subprog exit edge

2026-09-02T13:54:53+00:00

Nicholas Carlini reported a bug in precision backtracking mechanism
for BPF_LD | BPF_{IND,ABS} instructions. These instructions are
modelled as two branches:
- fallthrough;
- implicit exit from current subprogram.

The implicit exit case was not handled by the backtrack_insn()
function. When backtracking such a path backtrack_insn() did not
call bt_subprog_enter(), which meant that backtracking continued
manipulating precision marks in a caller frame, while looking at
instructions in a callee frame.

This lead to segmentation faults during verification (see the
selftest), or unsound state pruning.

Fixes: ee861486e377 ("bpf: Fix ld_{abs,ind} failure path analysis in subprogs")
Reported-by: Nicholas Carlini 
Suggested-by: Nicholas Carlini 
Signed-off-by: Eduard Zingerman 
Signed-off-by: Daniel Borkmann 
Acked-by: Daniel Borkmann 
Link: https://lore.kernel.org/bpf/20260901-bug-016-backtrack-ld-abs-v1-1-59368f1be435@gmail.com

bpf: Drop duplicate blank lines in kernel/bpf/

2026-08-13T17:27:40+00:00

There are many adjacent blank lines in kernel/bpf/ that have accumulated
over time.

Drop them for cleanup.

No functional changes intended.

Signed-off-by: Leon Hwang 
Signed-off-by: Andrii Nakryiko 
Reviewed-by: Emil Tsalapatis 
Link: https://lore.kernel.org/bpf/20260813152324.97937-2-leon.hwang@linux.dev

bpf: Do not print a newline after disassembly in bpf_verbose_insn()

2026-08-08T09:05:49+00:00

At the moment there are more callsites that want bpf_verbose_insn() to
not print a newline after the instruction, than callsites that want a
newline. Drop '\n' from disasm.c. Non-functional change.

The changes in bpftool are verified by writing a bpf program using a
variety of instructions and comparing `prog dump xlated` output in the
following modes: plain, opcodes, visual, visual opcodes. The output
before and after the changes is identical.

Signed-off-by: Eduard Zingerman 
Signed-off-by: Daniel Borkmann 
Reviewed-by: Quentin Monnet 
Acked-by: Daniel Borkmann 
Link: https://lore.kernel.org/bpf/20260807-static-zext-v4-1-b6c270013c77@gmail.com

bpf: Rename ARG_CONST_SIZE{,_OR_ZERO} to ARG_MEM_SIZE{,_OR_ZERO}

2026-08-02T22:29:14+00:00

ARG_CONST_SIZE does not require a constant: check_mem_size_reg() accepts
any bounded scalar and verifies the memory access against its maximum
(reg_umax). Rename ARG_CONST_SIZE and ARG_CONST_SIZE_OR_ZERO to
ARG_MEM_SIZE and ARG_MEM_SIZE_OR_ZERO to reflect that. ARG_CONST_ALLOC_
SIZE_OR_ZERO, which does require a constant, is left unchanged.

Pure rename, no functional change.

Signed-off-by: Amery Hung 
Link: https://lore.kernel.org/bpf/20260801074633.1595644-10-ameryhung@gmail.com
Signed-off-by: Kumar Kartikeya Dwivedi

bpf: Add precision marking and backtracking for stack argument slots

2026-05-13T16:27:30+00:00

Extend the precision marking and backtracking infrastructure to
support stack argument slots (r11-based accesses). Without this,
precision demands for scalar values passed through stack arguments
are silently dropped, which could allow the verifier to incorrectly
prune states with different constant values in stack arg slots.

Signed-off-by: Yonghong Song 
Link: https://lore.kernel.org/r/20260513045025.2387526-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov

bpf: Refactor jmp history to use dedicated spi/frame fields

2026-05-13T16:27:30+00:00

Move stack slot index (spi) and frame number out of the flags field
in bpf_jmp_history_entry into dedicated bitfields. This simplifies
the encoding and makes room for new flags.

Previously, spi and frame were packed into the lower 9 bits of the
12-bit flags field (3 bits frame + 6 bits spi), with INSN_F_STACK_ACCESS
at BIT(9) and INSN_F_DST/SRC_REG_STACK at BIT(10)/BIT(11).
But this has no room for an INSN_F_* flag for stack arguments.

To resolve this issue, bpf_jmp_history_entry field idx is narrowed to
20 bits (sufficient for insn indices up to 1M), and the freed bits hold
spi (6 bits) and frame (3 bits) as dedicated struct fields. The flags
enum is simplified accordingly:
  INSN_F_STACK_ACCESS  -> BIT(0)
  INSN_F_DST_REG_STACK -> BIT(1)
  INSN_F_SRC_REG_STACK -> BIT(2)
which allows more room for additional INSN_F_* flags.

bpf_push_jmp_history() now takes explicit spi and frame parameters
instead of encoding them into flags. The insn_stack_access_flags(),
insn_stack_access_spi(), and insn_stack_access_frameno() helpers are
removed.

No functional change.

Signed-off-by: Yonghong Song 
Link: https://lore.kernel.org/r/20260513045020.2385962-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov

bpf: Move backtracking logic to backtrack.c

2026-04-12T19:36:58+00:00

Move precision propagation and backtracking logic to backtrack.c
to reduce verifier.c size.

No functional changes.

Acked-by: Kumar Kartikeya Dwivedi 
Acked-by: Daniel Borkmann 
Link: https://lore.kernel.org/r/20260412152936.54262-6-alexei.starovoitov@gmail.com
Signed-off-by: Alexei Starovoitov