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Tianci Cao 4c03342e5a bpf: Add bitwise tracking for BPF_END 
[ Upstream commit 9d21199842 ]

This patch implements bitwise tracking (tnum analysis) for BPF_END
(byte swap) operation.

Currently, the BPF verifier does not track value for BPF_END operation,
treating the result as completely unknown. This limits the verifier's
ability to prove safety of programs that perform endianness conversions,
which are common in networking code.

For example, the following code pattern for port number validation:

int test(struct pt_regs *ctx) {
    __u64 x = bpf_get_prandom_u32();
    x &= 0x3f00;           // Range: [0, 0x3f00], var_off: (0x0; 0x3f00)
    x = bswap16(x);        // Should swap to range [0, 0x3f], var_off: (0x0; 0x3f)
    if (x > 0x3f) goto trap;
    return 0;
trap:
    return *(u64 *)NULL;   // Should be unreachable
}

Currently generates verifier output:

1: (54) w0 &= 16128                   ; R0=scalar(smin=smin32=0,smax=umax=smax32=umax32=16128,var_off=(0x0; 0x3f00))
2: (d7) r0 = bswap16 r0               ; R0=scalar()
3: (25) if r0 > 0x3f goto pc+2        ; R0=scalar(smin=smin32=0,smax=umax=smax32=umax32=63,var_off=(0x0; 0x3f))

Without this patch, even though the verifier knows `x` has certain bits
set, after bswap16, it loses all tracking information and treats port
as having a completely unknown value [0, 65535].

According to the BPF instruction set[1], there are 3 kinds of BPF_END:

1. `bswap(16|32|64)`: opcode=0xd7 (BPF_END | BPF_ALU64 | BPF_TO_LE)
   - do unconditional swap
2. `le(16|32|64)`: opcode=0xd4 (BPF_END | BPF_ALU | BPF_TO_LE)
   - on big-endian: do swap
   - on little-endian: truncation (16/32-bit) or no-op (64-bit)
3. `be(16|32|64)`: opcode=0xdc (BPF_END | BPF_ALU | BPF_TO_BE)
   - on little-endian: do swap
   - on big-endian: truncation (16/32-bit) or no-op (64-bit)

Since BPF_END operations are inherently bit-wise permutations, tnum
(bitwise tracking) offers the most efficient and precise mechanism
for value analysis. By implementing `tnum_bswap16`, `tnum_bswap32`,
and `tnum_bswap64`, we can derive exact `var_off` values concisely,
directly reflecting the bit-level changes.

Here is the overview of changes:

1. In `tnum_bswap(16|32|64)` (kernel/bpf/tnum.c):

Call `swab(16|32|64)` function on the value and mask of `var_off`, and
do truncation for 16/32-bit cases.

2. In `adjust_scalar_min_max_vals` (kernel/bpf/verifier.c):

Call helper function `scalar_byte_swap`.
- Only do byte swap when
  * alu64 (unconditional swap) OR
  * switching between big-endian and little-endian machines.
- If need do byte swap:
  * Firstly call `tnum_bswap(16|32|64)` to update `var_off`.
  * Then reset the bound since byte swap scrambles the range.
- For 16/32-bit cases, truncate dst register to match the swapped size.

This enables better verification of networking code that frequently uses
byte swaps for protocol processing, reducing false positive rejections.

[1] https://www.kernel.org/doc/Documentation/bpf/standardization/instruction-set.rst

Co-developed-by: Shenghao Yuan <shenghaoyuan0928@163.com>
Signed-off-by: Shenghao Yuan <shenghaoyuan0928@163.com>
Co-developed-by: Yazhou Tang <tangyazhou518@outlook.com>
Signed-off-by: Yazhou Tang <tangyazhou518@outlook.com>
Signed-off-by: Tianci Cao <ziye@zju.edu.cn>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260204111503.77871-2-ziye@zju.edu.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Stable-dep-of: efc11a6678 ("bpf: Improve bounds when tnum has a single possible value")
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2026-03-12 07:09:23 -04:00
arch bpf, arm64: Force 8-byte alignment for JIT buffer to prevent atomic tearing 2026-03-12 07:09:21 -04:00
block block: decouple secure erase size limit from discard size limit 2026-03-04 07:19:38 -05:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto crypto: authencesn - reject too-short AAD (assoclen<8) to match ESP/ESN spec 2026-01-30 10:32:12 +01:00
Documentation ipv6: Move ipv6_fl_list from ipv6_pinfo to inet_sock. 2026-03-04 07:20:44 -05:00
drivers spi: stm32: fix missing pointer assignment in case of dma chaining 2026-03-12 07:09:22 -04:00
fs btrfs: fix compat mask in error messages in btrfs_check_features() 2026-03-12 07:09:20 -04:00
include bpf: Add bitwise tracking for BPF_END 2026-03-12 07:09:23 -04:00
init printk changes for 6.18 2025-10-04 11:13:11 -07:00
io_uring io_uring/zcrx: fix user_ref race between scrub and refill paths 2026-03-04 07:21:33 -05:00
ipc ipc: don't audit capability check in ipc_permissions() 2026-02-26 14:59:19 -08:00
kernel bpf: Add bitwise tracking for BPF_END 2026-03-12 07:09:23 -04:00
lib debugobject: Make it work with deferred page initialization - again 2026-03-12 07:09:08 -04:00
LICENSES LICENSES: Replace the obsolete address of the FSF in the GFDL-1.2 2025-07-24 11:15:39 +02:00
mm mm: numa_memblks: Identify the accurate NUMA ID of CFMW 2026-03-04 07:21:31 -05:00
net net: nfc: nci: Fix parameter validation for packet data 2026-03-04 07:21:34 -05:00
rust rust: list: Add unsafe blocks for container_of and safety comments 2026-03-04 07:21:35 -05:00
samples samples/ftrace: Adjust LoongArch register restore order in direct calls 2026-01-08 10:17:11 +01:00
scripts kbuild: rpm-pkg: Disable automatic requires for manual debuginfo package 2026-03-04 07:21:32 -05:00
security ima: verify the previous kernel's IMA buffer lies in addressable RAM 2026-03-04 07:21:17 -05:00
sound ASoC: SDCA: Fix comments for sdca_irq_request() 2026-03-12 07:09:21 -04:00
tools selftests/bpf: Fix OOB read in dmabuf_collector 2026-03-12 07:09:21 -04:00
usr gen_init_cpio: Ignore fsync() returning EINVAL on pipes 2025-10-07 09:53:05 -07:00
virt KVM: Don't clobber irqfd routing type when deassigning irqfd 2026-02-11 13:41:44 +01:00
.clang-format memblock: drop for_each_free_mem_pfn_range_in_zone_from() 2025-09-14 08:49:03 +03:00
.clippy.toml rust: clean Rust 1.88.0's warning about clippy::disallowed_macros configuration 2025-05-07 00:11:47 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore MAINTAINERS: remove Alyssa Rosenzweig 2025-09-18 21:17:31 +02:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore .gitignore: ignore compile_commands.json globally 2025-08-12 15:53:55 -07:00
.mailmap 8 hotfixes. 4 are cc:stable, 7 are against mm/. 2025-11-26 12:38:05 -08:00
.pylintrc tools: docs: parse-headers.py: move it from sphinx dir 2025-08-29 15:54:42 -06:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: mark ISDN subsystem as orphan 2025-10-27 17:49:45 -07:00
Kbuild sched: Make migrate_{en,dis}able() inline 2025-09-25 09:57:16 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS soc: fixes for 6.18, part 4 2025-11-28 09:57:31 -08:00
Makefile Linux 6.18.16 2026-03-04 07:25:12 -05:00
README README: Fix spelling 2024-03-18 03:36:32 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.