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Alexei Starovoitov 6e951a9b1a Merge branch 'bpf-improve-linked-register-tracking'
Puranjay Mohan says:

====================
bpf: Improve linked register tracking

V3: https://lore.kernel.org/all/20260203222643.994713-1-puranjay@kernel.org/
Changes in v3->v4:
- Add a call to reg_bounds_sync() in sync_linked_regs() to sync bounds after alu op.
- Add a __sink(path[0]); in the C selftest so compiler doesn't say  "error: variable 'path' set but
  not used"

V2: https://lore.kernel.org/all/20260113152529.3217648-1-puranjay@kernel.org/
Changes in v2->v3:
- Added another selftest showing a real usage pattern
- Rebased on bpf-next/master

v1: https://lore.kernel.org/bpf/20260107203941.1063754-1-puranjay@kernel.org/
Changes in v1->v2:
- Add support for alu32 operations in linked register tracking (Alexei)
- Squash the selftest fix with the first patch (Eduard)
- Add more selftests to detect edge cases

This series extends the BPF verifier's linked register tracking to handle
negative offsets, BPF_SUB operations, and alu32 operations, enabling better bounds propagation for
common arithmetic patterns.

The verifier previously only tracked positive constant deltas between linked
registers using BPF_ADD. This meant patterns using negative offsets or
subtraction couldn't benefit from bounds propagation:

  void alu32_negative_offset(void)
  {
          volatile char path[5];
          volatile int offset = bpf_get_prandom_u32();
          int off = offset;

          if (off >= 5 && off < 10)
                  path[off - 5] = '.';
  }

this gets compiled to:

0000000000000478 <alu32_negative_offset>:
     143:       call 0x7
     144:       *(u32 *)(r10 - 0xc) = w0
     145:       w1 = *(u32 *)(r10 - 0xc)
     146:       w2 = w1				// w2 and w1 share the same id
     147:       w2 += -0x5			// verifier knows w1 = w2 + 5
     148:       if w2 > 0x4 goto +0x5 <L0>	// in fall-through: verifier knows w2 ∈ [0,4] => w1 ∈ [5, 9]
     149:       r2 = r10
     150:       r2 += -0x5			// r2 = fp - 5
     151:       r2 += r1			// r2 = fp - 5 + r1 (∈ [5, 9]) => r2 ∈ [fp, fp + 4]
     152:       w1 = 0x2e
     153:       *(u8 *)(r2 - 0x5) = w1		// r2 ∈ [fp, fp + 4] => r2 - 5 ∈ [fp - 5, fp - 1]
<L0>:
     154:       exit

After the changes, the verifier could link 32-bit scalars and also supported -ve offsets for linking:

	146:       w2 = w1
	147:       w2 += -0x5

It allowed the verifier to correctly propagate bounds, without the
changes in this patchset, verifier would reject this program with:

invalid unbounded variable-offset write to stack R2

This program has been added as a selftest in the second patch.

Veristat comparison on programs from sched_ext, selftests, and some meta internal programs:

Scx Progs
File               Program           Verdict (A)  Verdict (B)  Verdict (DIFF)  Insns (A)  Insns (B)  Insns  (DIFF)
-----------------  ----------------  -----------  -----------  --------------  ---------  ---------  -------------
scx_layered.bpf.o  layered_runnable  success      success      MATCH                5674       6077  +403 (+7.10%)

FB Progs
File          Program           Verdict (A)  Verdict (B)  Verdict (DIFF)  Insns (A)  Insns (B)  Insns      (DIFF)
------------  ----------------  -----------  -----------  --------------  ---------  ---------  -----------------
bpf232.bpf.o  layered_dump      success      success      MATCH                1151       1218       +67 (+5.82%)
bpf257.bpf.o  layered_runnable  success      success      MATCH                5743       6143      +400 (+6.97%)
bpf252.bpf.o  layered_runnable  success      success      MATCH                5677       6075      +398 (+7.01%)
bpf227.bpf.o  layered_dump      success      success      MATCH                 915        982       +67 (+7.32%)
bpf239.bpf.o  layered_runnable  success      success      MATCH                5459       5861      +402 (+7.36%)
bpf246.bpf.o  layered_runnable  success      success      MATCH                5562       6008      +446 (+8.02%)
bpf229.bpf.o  layered_runnable  success      success      MATCH                2559       3011     +452 (+17.66%)
bpf231.bpf.o  layered_runnable  success      success      MATCH                2559       3011     +452 (+17.66%)
bpf234.bpf.o  layered_runnable  success      success      MATCH                2549       3001     +452 (+17.73%)
bpf019.bpf.o  do_sendmsg        success      success      MATCH              124823     153523   +28700 (+22.99%)
bpf019.bpf.o  do_parse          success      success      MATCH              124809     153509   +28700 (+23.00%)
bpf227.bpf.o  layered_runnable  success      success      MATCH                1915       2356     +441 (+23.03%)
bpf228.bpf.o  layered_runnable  success      success      MATCH                1700       2152     +452 (+26.59%)
bpf232.bpf.o  layered_runnable  success      success      MATCH                1499       1951     +452 (+30.15%)

bpf312.bpf.o  mount_exit        success      success      MATCH               19253      62883  +43630 (+226.61%)
bpf312.bpf.o  umount_exit       success      success      MATCH               19253      62883  +43630 (+226.61%)
bpf311.bpf.o  mount_exit        success      success      MATCH               19226      62863  +43637 (+226.97%)
bpf311.bpf.o  umount_exit       success      success      MATCH               19226      62863  +43637 (+226.97%)

The above four programs have specific patters that make the verifier explore a lot more states:

    for (; depth < MAX_DIR_DEPTH; depth++) {
        const unsigned char* name = BPF_CORE_READ(dentry, d_name.name);
        if (offset >= MAX_PATH_LEN - MAX_DIR_LEN) {
            return depth;
        }
        int len = bpf_probe_read_kernel_str(&path[offset], MAX_DIR_LEN, name);
        offset += len;
        if (len == MAX_DIR_LEN) {
            if (offset - 2 < MAX_PATH_LEN) {   // <---- (a)
                path[offset - 2] = '.';
            }
            if (offset - 3 < MAX_PATH_LEN) {   // <---- (b)
                path[offset - 3] = '.';
            }
            if (offset - 4 < MAX_PATH_LEN) {   // <---- (c)
                path[offset - 4] = '.';
            }
        }
    }

When at some depth == N false branches of conditions (a), (b) and (c) are scheduled for
verification, constraints for offset at depth == N+1 are:
1. offset >= MAX_PATH_LEN + 2
2. offset >= MAX_PATH_LEN + 3
3. offset >= MAX_PATH_LEN + 4 (visited before others)

And after offset += len it becomes:
1. offset >= MAX_PATH_LEN - 4093
2. offset >= MAX_PATH_LEN - 4092
3. offset >= MAX_PATH_LEN - 4091 (visited before others)

Because of the DFS states exploration logic, the states above are visited in order 3, 2, 1; 3 is not
a subset of 2 and 1 is not a subset of 2, so pruning logic does not kick in.
Previously this was not a problem, because range for offset was not propagated through the
statements (a), (b), (c).

As the root cause of this regression is understood, this is not a blocker for this change.

Selftest Progs
File                                Program                   Verdict (A)  Verdict (B)  Verdict (DIFF)  Insns (A)  Insns (B)  Insns   (DIFF)
----------------------------------  ------------------------  -----------  -----------  --------------  ---------  ---------  --------------
linked_list_peek.bpf.o              list_peek                 success      success      MATCH                 152         88   -64 (-42.11%)
verifier_iterating_callbacks.bpf.o  cond_break2               success      success      MATCH                 110         88   -22 (-20.00%)

These are the added selftests that failed earlier but are passing now:

verifier_linked_scalars.bpf.o       alu32_negative_offset     failure      success      MISMATCH               11         13    +2 (+18.18%)
verifier_linked_scalars.bpf.o       scalars_alu32_big_offset  failure      success      MISMATCH                7         10    +3 (+42.86%)
verifier_linked_scalars.bpf.o       scalars_neg_alu32_add     failure      success      MISMATCH                7         10    +3 (+42.86%)
verifier_linked_scalars.bpf.o       scalars_neg_alu32_sub     failure      success      MISMATCH                7         10    +3 (+42.86%)
verifier_linked_scalars.bpf.o       scalars_neg               failure      success      MISMATCH                7         10    +3 (+42.86%)
verifier_linked_scalars.bpf.o       scalars_neg_sub           failure      success      MISMATCH                7         10    +3 (+42.86%)
verifier_linked_scalars.bpf.o       scalars_sub_neg_imm       failure      success      MISMATCH                7         10    +3 (+42.86%)

iters.bpf.o                         iter_obfuscate_counter    success      success      MATCH                  83        119   +36 (+43.37%)
bpf_cubic.bpf.o                     bpf_cubic_acked           success      success      MATCH                 243        430  +187 (+76.95%)
====================

Link: https://patch.msgid.link/20260204151741.2678118-1-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
History 2026-02-04 13:35:29 -08:00
arch bpf, arm64: Add fsession support 2026-01-31 13:51:04 -08:00
block block-6.19-20260109 2026-01-09 15:42:46 -10:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto crypto: seqiv - Do not use req->iv after crypto_aead_encrypt 2025-12-19 14:47:06 +08:00
Documentation bpf,docs: Document KF_IMPLICIT_ARGS flag 2026-01-20 16:22:38 -08:00
drivers HID: Use bpf_wq_set_callback kernel function 2026-01-20 16:15:57 -08:00
fs Merge git://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf after rc5 2026-01-14 15:22:01 -08:00
include bpf: Support negative offsets, BPF_SUB, and alu32 for linked register tracking 2026-02-04 13:35:28 -08:00
init kbuild: Sync kconfig when PAHOLE_VERSION changes 2025-12-19 10:55:40 -08:00
io_uring io_uring-6.19-20260109 2026-01-09 15:21:15 -10:00
ipc Significant patch series in this pull request: 2025-12-06 14:01:20 -08:00
kernel bpf: Support negative offsets, BPF_SUB, and alu32 for linked register tracking 2026-02-04 13:35:28 -08:00
lib Merge git://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf after rc5 2026-01-14 15:22:01 -08:00
LICENSES LICENSES: Add modern form of the LGPL-2.1 tags to the usage guide section 2025-10-22 07:58:19 +02:00
mm bpf: Revert "bpf: drop KF_ACQUIRE flag on BPF kfunc bpf_get_root_mem_cgroup()" 2026-01-21 09:38:16 -08:00
net bpf: Don't check sk_fullsock() in bpf_skc_to_unix_sock(). 2026-02-04 09:36:06 -08:00
rust Driver core fixes for 6.19-rc5 2026-01-10 15:04:04 -10:00
samples samples/ftrace: Adjust LoongArch register restore order in direct calls 2025-12-31 15:19:25 +08:00
scripts scripts/gen-btf.sh: Use CONFIG_SHELL for execution 2026-01-21 12:36:32 -08:00
security kernel/kexec: change the prototype of kimage_map_segment() 2025-12-23 11:23:13 -08:00
sound ALSA: hda/realtek: add HP Laptop 15s-eq1xxx mute LED quirk 2026-01-02 14:34:23 +01:00
tools selftests/bpf: Add tests for improved linked register tracking 2026-02-04 13:35:29 -08:00
usr initramfs: add gen_init_cpio to hostprogs unconditionally 2025-11-26 21:55:40 +01:00
virt KVM fixes for 6.19-rc1 2025-12-18 18:38:45 +01:00
.clang-format s390/pci: Fix cyclic dead-lock in zpci_zdev_put() and zpci_scan_devices() 2025-12-14 11:03:58 +01:00
.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 rust: kbuild: add proc macro library support 2025-11-24 17:15:36 +01:00
.mailmap treewide: Update email address 2026-01-11 06:09:11 -10:00
.pylintrc docs: Move the python libraries to tools/lib/python 2025-11-18 09:22:40 -07:00
.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 treewide: Update email address 2026-01-11 06:09:11 -10:00
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 Merge git://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf after rc5 2026-01-14 15:22:01 -08:00
Makefile Merge git://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf after rc5 2026-01-14 15:22:01 -08:00
README README: restructure with role-based documentation and guidelines 2025-11-29 08:40:33 -07:00

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

The Linux kernel is the core of any Linux operating system. It manages hardware,
system resources, and provides the fundamental services for all other software.

Quick Start
-----------

* Report a bug: See Documentation/admin-guide/reporting-issues.rst
* Get the latest kernel: https://kernel.org
* Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst
* Join the community: https://lore.kernel.org/

Essential Documentation
-----------------------

All users should be familiar with:

* Building requirements: Documentation/process/changes.rst
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* License: See COPYING

Documentation can be built with make htmldocs or viewed online at:
https://www.kernel.org/doc/html/latest/


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============

Find your role below:

* New Kernel Developer - Getting started with kernel development
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For Specific Users
==================

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--------------------

Welcome! Start your kernel development journey here:

* Getting Started: Documentation/process/development-process.rst
* Your First Patch: Documentation/process/submitting-patches.rst
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-------------------

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* Researcher Guidelines: Documentation/process/researcher-guidelines.rst
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* Scheduler: Documentation/scheduler/index.rst
* Networking Stack: Documentation/networking/index.rst
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---------------

Security documentation and hardening guides:

* Security Documentation: Documentation/security/index.rst
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-----------------------------

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* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* Backporting Guide: Documentation/process/backporting.rst
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--------------------

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* Admin Guide: Documentation/admin-guide/index.rst
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----------

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---------------

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-----------------------

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* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* ABI Documentation: Documentation/ABI/README
* Kernel Configuration: Documentation/kbuild/kconfig.rst
* Module Signing: Documentation/admin-guide/module-signing.rst
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* Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst



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=========================

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