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Andrii Nakryiko 1c1f9d299c
 BACKPORT: bpf: Switch most helper return values from 32-bit int to 64-bit long 
Switch most of BPF helper definitions from returning int to long. These
definitions are coming from comments in BPF UAPI header and are used to
generate bpf_helper_defs.h (under libbpf) to be later included and used from
BPF programs.

In actual in-kernel implementation, all the helpers are defined as returning
u64, but due to some historical reasons, most of them are actually defined as
returning int in UAPI (usually, to return 0 on success, and negative value on
error).

This actually causes Clang to quite often generate sub-optimal code, because
compiler believes that return value is 32-bit, and in a lot of cases has to be
up-converted (usually with a pair of 32-bit bit shifts) to 64-bit values,
before they can be used further in BPF code.

Besides just "polluting" the code, these 32-bit shifts quite often cause
problems for cases in which return value matters. This is especially the case
for the family of bpf_probe_read_str() functions. There are few other similar
helpers (e.g., bpf_read_branch_records()), in which return value is used by
BPF program logic to record variable-length data and process it. For such
cases, BPF program logic carefully manages offsets within some array or map to
read variable-length data. For such uses, it's crucial for BPF verifier to
track possible range of register values to prove that all the accesses happen
within given memory bounds. Those extraneous zero-extending bit shifts,
inserted by Clang (and quite often interleaved with other code, which makes
the issues even more challenging and sometimes requires employing extra
per-variable compiler barriers), throws off verifier logic and makes it mark
registers as having unknown variable offset. We'll study this pattern a bit
later below.

Another common pattern is to check return of BPF helper for non-zero state to
detect error conditions and attempt alternative actions in such case. Even in
this simple and straightforward case, this 32-bit vs BPF's native 64-bit mode
quite often leads to sub-optimal and unnecessary extra code. We'll look at
this pattern as well.

Clang's BPF target supports two modes of code generation: ALU32, in which it
is capable of using lower 32-bit parts of registers, and no-ALU32, in which
only full 64-bit registers are being used. ALU32 mode somewhat mitigates the
above described problems, but not in all cases.

This patch switches all the cases in which BPF helpers return 0 or negative
error from returning int to returning long. It is shown below that such change
in definition leads to equivalent or better code. No-ALU32 mode benefits more,
but ALU32 mode doesn't degrade or still gets improved code generation.

Another class of cases switched from int to long are bpf_probe_read_str()-like
helpers, which encode successful case as non-negative values, while still
returning negative value for errors.

In all of such cases, correctness is preserved due to two's complement
encoding of negative values and the fact that all helpers return values with
32-bit absolute value. Two's complement ensures that for negative values
higher 32 bits are all ones and when truncated, leave valid negative 32-bit
value with the same value. Non-negative values have upper 32 bits set to zero
and similarly preserve value when high 32 bits are truncated. This means that
just casting to int/u32 is correct and efficient (and in ALU32 mode doesn't
require any extra shifts).

To minimize the chances of regressions, two code patterns were investigated,
as mentioned above. For both patterns, BPF assembly was analyzed in
ALU32/NO-ALU32 compiler modes, both with current 32-bit int return type and
new 64-bit long return type.

Case 1. Variable-length data reading and concatenation. This is quite
ubiquitous pattern in tracing/monitoring applications, reading data like
process's environment variables, file path, etc. In such case, many pieces of
string-like variable-length data are read into a single big buffer, and at the
end of the process, only a part of array containing actual data is sent to
user-space for further processing. This case is tested in test_varlen.c
selftest (in the next patch). Code flow is roughly as follows:

  void *payload = &sample->payload;
  u64 len;

  len = bpf_probe_read_kernel_str(payload, MAX_SZ1, &source_data1);
  if (len <= MAX_SZ1) {
      payload += len;
      sample->len1 = len;
  }
  len = bpf_probe_read_kernel_str(payload, MAX_SZ2, &source_data2);
  if (len <= MAX_SZ2) {
      payload += len;
      sample->len2 = len;
  }
  /* and so on */
  sample->total_len = payload - &sample->payload;
  /* send over, e.g., perf buffer */

There could be two variations with slightly different code generated: when len
is 64-bit integer and when it is 32-bit integer. Both variations were analysed.
BPF assembly instructions between two successive invocations of
bpf_probe_read_kernel_str() were used to check code regressions. Results are
below, followed by short analysis. Left side is using helpers with int return
type, the right one is after the switch to long.

ALU32 + INT                                ALU32 + LONG
===========                                ============

64-BIT (13 insns):                         64-BIT (10 insns):
------------------------------------       ------------------------------------
  17:   call 115                             17:   call 115
  18:   if w0 > 256 goto +9 <LBB0_4>         18:   if r0 > 256 goto +6 <LBB0_4>
  19:   w1 = w0                              19:   r1 = 0 ll
  20:   r1 <<= 32                            21:   *(u64 *)(r1 + 0) = r0
  21:   r1 s>>= 32                           22:   r6 = 0 ll
  22:   r2 = 0 ll                            24:   r6 += r0
  24:   *(u64 *)(r2 + 0) = r1              00000000000000c8 <LBB0_4>:
  25:   r6 = 0 ll                            25:   r1 = r6
  27:   r6 += r1                             26:   w2 = 256
00000000000000e0 <LBB0_4>:                   27:   r3 = 0 ll
  28:   r1 = r6                              29:   call 115
  29:   w2 = 256
  30:   r3 = 0 ll
  32:   call 115

32-BIT (11 insns):                         32-BIT (12 insns):
------------------------------------       ------------------------------------
  17:   call 115                             17:   call 115
  18:   if w0 > 256 goto +7 <LBB1_4>         18:   if w0 > 256 goto +8 <LBB1_4>
  19:   r1 = 0 ll                            19:   r1 = 0 ll
  21:   *(u32 *)(r1 + 0) = r0                21:   *(u32 *)(r1 + 0) = r0
  22:   w1 = w0                              22:   r0 <<= 32
  23:   r6 = 0 ll                            23:   r0 >>= 32
  25:   r6 += r1                             24:   r6 = 0 ll
00000000000000d0 <LBB1_4>:                   26:   r6 += r0
  26:   r1 = r6                            00000000000000d8 <LBB1_4>:
  27:   w2 = 256                             27:   r1 = r6
  28:   r3 = 0 ll                            28:   w2 = 256
  30:   call 115                             29:   r3 = 0 ll
                                             31:   call 115

In ALU32 mode, the variant using 64-bit length variable clearly wins and
avoids unnecessary zero-extension bit shifts. In practice, this is even more
important and good, because BPF code won't need to do extra checks to "prove"
that payload/len are within good bounds.

32-bit len is one instruction longer. Clang decided to do 64-to-32 casting
with two bit shifts, instead of equivalent `w1 = w0` assignment. The former
uses extra register. The latter might potentially lose some range information,
but not for 32-bit value. So in this case, verifier infers that r0 is [0, 256]
after check at 18:, and shifting 32 bits left/right keeps that range intact.
We should probably look into Clang's logic and see why it chooses bitshifts
over sub-register assignments for this.

NO-ALU32 + INT                             NO-ALU32 + LONG
==============                             ===============

64-BIT (14 insns):                         64-BIT (10 insns):
------------------------------------       ------------------------------------
  17:   call 115                             17:   call 115
  18:   r0 <<= 32                            18:   if r0 > 256 goto +6 <LBB0_4>
  19:   r1 = r0                              19:   r1 = 0 ll
  20:   r1 >>= 32                            21:   *(u64 *)(r1 + 0) = r0
  21:   if r1 > 256 goto +7 <LBB0_4>         22:   r6 = 0 ll
  22:   r0 s>>= 32                           24:   r6 += r0
  23:   r1 = 0 ll                          00000000000000c8 <LBB0_4>:
  25:   *(u64 *)(r1 + 0) = r0                25:   r1 = r6
  26:   r6 = 0 ll                            26:   r2 = 256
  28:   r6 += r0                             27:   r3 = 0 ll
00000000000000e8 <LBB0_4>:                   29:   call 115
  29:   r1 = r6
  30:   r2 = 256
  31:   r3 = 0 ll
  33:   call 115

32-BIT (13 insns):                         32-BIT (13 insns):
------------------------------------       ------------------------------------
  17:   call 115                             17:   call 115
  18:   r1 = r0                              18:   r1 = r0
  19:   r1 <<= 32                            19:   r1 <<= 32
  20:   r1 >>= 32                            20:   r1 >>= 32
  21:   if r1 > 256 goto +6 <LBB1_4>         21:   if r1 > 256 goto +6 <LBB1_4>
  22:   r2 = 0 ll                            22:   r2 = 0 ll
  24:   *(u32 *)(r2 + 0) = r0                24:   *(u32 *)(r2 + 0) = r0
  25:   r6 = 0 ll                            25:   r6 = 0 ll
  27:   r6 += r1                             27:   r6 += r1
00000000000000e0 <LBB1_4>:                 00000000000000e0 <LBB1_4>:
  28:   r1 = r6                              28:   r1 = r6
  29:   r2 = 256                             29:   r2 = 256
  30:   r3 = 0 ll                            30:   r3 = 0 ll
  32:   call 115                             32:   call 115

In NO-ALU32 mode, for the case of 64-bit len variable, Clang generates much
superior code, as expected, eliminating unnecessary bit shifts. For 32-bit
len, code is identical.

So overall, only ALU-32 32-bit len case is more-or-less equivalent and the
difference stems from internal Clang decision, rather than compiler lacking
enough information about types.

Case 2. Let's look at the simpler case of checking return result of BPF helper
for errors. The code is very simple:

  long bla;
  if (bpf_probe_read_kenerl(&bla, sizeof(bla), 0))
      return 1;
  else
      return 0;

ALU32 + CHECK (9 insns)                    ALU32 + CHECK (9 insns)
====================================       ====================================
  0:    r1 = r10                             0:    r1 = r10
  1:    r1 += -8                             1:    r1 += -8
  2:    w2 = 8                               2:    w2 = 8
  3:    r3 = 0                               3:    r3 = 0
  4:    call 113                             4:    call 113
  5:    w1 = w0                              5:    r1 = r0
  6:    w0 = 1                               6:    w0 = 1
  7:    if w1 != 0 goto +1 <LBB2_2>          7:    if r1 != 0 goto +1 <LBB2_2>
  8:    w0 = 0                               8:    w0 = 0
0000000000000048 <LBB2_2>:                 0000000000000048 <LBB2_2>:
  9:    exit                                 9:    exit

Almost identical code, the only difference is the use of full register
assignment (r1 = r0) vs half-registers (w1 = w0) in instruction #5. On 32-bit
architectures, new BPF assembly might be slightly less optimal, in theory. But
one can argue that's not a big issue, given that use of full registers is
still prevalent (e.g., for parameter passing).

NO-ALU32 + CHECK (11 insns)                NO-ALU32 + CHECK (9 insns)
====================================       ====================================
  0:    r1 = r10                             0:    r1 = r10
  1:    r1 += -8                             1:    r1 += -8
  2:    r2 = 8                               2:    r2 = 8
  3:    r3 = 0                               3:    r3 = 0
  4:    call 113                             4:    call 113
  5:    r1 = r0                              5:    r1 = r0
  6:    r1 <<= 32                            6:    r0 = 1
  7:    r1 >>= 32                            7:    if r1 != 0 goto +1 <LBB2_2>
  8:    r0 = 1                               8:    r0 = 0
  9:    if r1 != 0 goto +1 <LBB2_2>        0000000000000048 <LBB2_2>:
 10:    r0 = 0                               9:    exit
0000000000000058 <LBB2_2>:
 11:    exit

NO-ALU32 is a clear improvement, getting rid of unnecessary zero-extension bit
shifts.

Change-Id: Ie4ba541847b25df0d89042e74ee18c7b3d99d091
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200623032224.4020118-1-andriin@fb.com
2026-01-14 17:50:46 -08:00
android Merge tag 'LA.UM.9.14.r1-26500-LAHAINA.QSSI16.0' of https://git.codelinaro.org/clo/la/kernel/msm-5.4 into android13-5.4-lahaina 2025-11-08 16:13:44 +02:00
arch BACKPORT: mm: remove the pgprot argument to __vmalloc 2026-01-14 17:48:09 -08:00
block Merge tag 'ASB-2025-12-01_11-5.4' of https://android.googlesource.com/kernel/common into android13-5.4-lahaina 2025-12-04 19:21:35 +02:00
certs
crypto BACKPORT: crypto: lib/sha1 - rename "sha" to "sha1" 2025-12-23 13:36:17 -08:00
Documentation UPSTREAM: bpf/flow_dissector: add mode to enforce global BPF flow dissector 2026-01-14 17:48:07 -08:00
drivers BACKPORT: mm: remove the pgprot argument to __vmalloc 2026-01-14 17:48:09 -08:00
fs BACKPORT: mm: remove the pgprot argument to __vmalloc 2026-01-14 17:48:09 -08:00
include BACKPORT: bpf: Switch most helper return values from 32-bit int to 64-bit long 2026-01-14 17:50:46 -08:00
init BACKPORT: bpf: Introduce BPF_PROG_TYPE_LSM 2025-12-23 13:36:09 -08:00
ipc BACKPORT: sysctl: pass kernel pointers to ->proc_handler 2025-12-23 13:36:15 -08:00
kernel UPSTREAM: bpf: Set map_btf_{name, id} for all map types 2026-01-14 17:50:46 -08:00
lib BACKPORT: crypto: lib/sha1 - rename "sha" to "sha1" 2025-12-23 13:36:17 -08:00
LICENSES
mm BACKPORT: mm: remove the pgprot argument to __vmalloc 2026-01-14 17:48:09 -08:00
net UPSTREAM: bpf: Set map_btf_{name, id} for all map types 2026-01-14 17:50:46 -08:00
samples UPSTREAM: devmap: Adjust tracepoint for map-less queue flush 2025-12-23 13:35:54 -08:00
scripts UPSTREAM: bpf: Minor fixes to BPF helpers documentation 2025-12-23 13:36:21 -08:00
security UPSTREAM: bpf, capability: Introduce CAP_BPF 2026-01-14 17:44:17 -08:00
sound BACKPORT: mm: remove the pgprot argument to __vmalloc 2026-01-14 17:48:09 -08:00
techpack Merge tag 'LA.UM.9.14.r1-26000-LAHAINA.QSSI15.0' of https://git.codelinaro.org/clo/la/platform/vendor/opensource/display-drivers into android13-5.4-lahaina 2025-04-12 09:35:58 +00:00
tools BACKPORT: bpf: Switch most helper return values from 32-bit int to 64-bit long 2026-01-14 17:50:46 -08:00
usr FROMLIST: kbuild: Add environment variables for userprogs flags 2025-09-28 10:00:32 +02:00
virt KVM: arm64: Ignore PMCNTENSET_EL0 while checking for overflow status 2024-12-19 18:05:04 +01:00
.clang-format
.cocciconfig
.get_maintainer.ignore
.gitattributes
.gitignore Merge android11-5.4.289(4c8fb32) into msm-5.4 2025-03-26 11:56:22 +05:30
.mailmap
Android.mk
Androidbp
build.config.aarch64
build.config.allmodconfig
build.config.allmodconfig.aarch64
build.config.allmodconfig.arm
build.config.allmodconfig.x86_64
build.config.arm
build.config.common
build.config.db845c
build.config.gki
build.config.gki-debug.aarch64
build.config.gki-debug.x86_64
build.config.gki.aarch64 ANDROID: GKI: add Trimble symbol list 2025-01-16 06:41:15 -08:00
build.config.gki.x86_64
build.config.gki_kasan
build.config.gki_kasan.aarch64
build.config.gki_kasan.x86_64
build.config.gki_kprobes
build.config.gki_kprobes.aarch64
build.config.gki_kprobes.x86_64
build.config.hikey960
build.config.msm.common
build.config.msm.gki
build.config.msm.lahaina
build.config.x86_64
build.targets
COPYING
CREDITS
gen_headers_arm.bp
gen_headers_arm64.bp
gen_headers_arm64_auto.bp
gen_headers_arm_auto.bp
Kbuild
Kconfig
kernel_headers.py
LICENSE
MAINTAINERS BACKPORT: bpf: Introduce BPF_PROG_TYPE_LSM 2025-12-23 13:36:09 -08:00
Makefile Merge branch 'android11-5.4-lts' of https://android.googlesource.com/kernel/common into android13-5.4-lahaina 2025-12-11 01:41:51 -08:00
modules.list.msm.lahaina
OWNERS
README
README.md

README.md

How do I submit patches to Android Common Kernels

  1. BEST: Make all of your changes to upstream Linux. If appropriate, backport to the stable releases. These patches will be merged automatically in the corresponding common kernels. If the patch is already in upstream Linux, post a backport of the patch that conforms to the patch requirements below.

  2. LESS GOOD: Develop your patches out-of-tree (from an upstream Linux point-of-view). Unless these are fixing an Android-specific bug, these are very unlikely to be accepted unless they have been coordinated with kernel-team@android.com. If you want to proceed, post a patch that conforms to the patch requirements below.

Common Kernel patch requirements

  • All patches must conform to the Linux kernel coding standards and pass script/checkpatch.pl
  • Patches shall not break gki_defconfig or allmodconfig builds for arm, arm64, x86, x86_64 architectures (see https://source.android.com/setup/build/building-kernels)
  • If the patch is not merged from an upstream branch, the subject must be tagged with the type of patch: UPSTREAM:, BACKPORT:, FROMGIT:, FROMLIST:, or ANDROID:.
  • All patches must have a Change-Id: tag (see https://gerrit-review.googlesource.com/Documentation/user-changeid.html)
  • If an Android bug has been assigned, there must be a Bug: tag.
  • All patches must have a Signed-off-by: tag by the author and the submitter

Additional requirements are listed below based on patch type

Requirements for backports from mainline Linux: UPSTREAM:, BACKPORT:

  • If the patch is a cherry-pick from Linux mainline with no changes at all
    • tag the patch subject with UPSTREAM:.
    • add upstream commit information with a (cherry-picked from ...) line
    • Example:
      • if the upstream commit message is
        important patch from upstream

        This is the detailed description of the important patch

        Signed-off-by: Fred Jones <fred.jones@foo.org>

    - then Joe Smith would upload the patch for the common kernel as

        UPSTREAM: important patch from upstream

        This is the detailed description of the important patch

        Signed-off-by: Fred Jones <fred.jones@foo.org>

        Bug: 135791357
        Change-Id: I4caaaa566ea080fa148c5e768bb1a0b6f7201c01
        (cherry-picked from c31e73121f4c1ec41143423ac6ce3ce6dafdcec1)
        Signed-off-by: Joe Smith <joe.smith@foo.org>
  • If the patch requires any changes from the upstream version, tag the patch with BACKPORT: instead of UPSTREAM:.
    • use the same tags as UPSTREAM:
    • add comments about the changes under the (cherry-picked from ...) line
    • Example:
        BACKPORT: important patch from upstream

        This is the detailed description of the important patch

        Signed-off-by: Fred Jones <fred.jones@foo.org>

        Bug: 135791357
        Change-Id: I4caaaa566ea080fa148c5e768bb1a0b6f7201c01
        (cherry-picked from c31e73121f4c1ec41143423ac6ce3ce6dafdcec1)
        [ Resolved minor conflict in drivers/foo/bar.c ]
        Signed-off-by: Joe Smith <joe.smith@foo.org>

Requirements for other backports: FROMGIT:, FROMLIST:,

  • If the patch has been merged into an upstream maintainer tree, but has not yet been merged into Linux mainline
    • tag the patch subject with FROMGIT:
    • add info on where the patch came from as (cherry picked from commit <sha1> <repo> <branch>). This must be a stable maintainer branch (not rebased, so don't use linux-next for example).
    • if changes were required, use BACKPORT: FROMGIT:
    • Example:
      • if the commit message in the maintainer tree is
        important patch from upstream

        This is the detailed description of the important patch

        Signed-off-by: Fred Jones <fred.jones@foo.org>

    - then Joe Smith would upload the patch for the common kernel as

        FROMGIT: important patch from upstream

        This is the detailed description of the important patch

        Signed-off-by: Fred Jones <fred.jones@foo.org>

        Bug: 135791357
        (cherry picked from commit 878a2fd9de10b03d11d2f622250285c7e63deace
         https://git.kernel.org/pub/scm/linux/kernel/git/foo/bar.git test-branch)
        Change-Id: I4caaaa566ea080fa148c5e768bb1a0b6f7201c01
        Signed-off-by: Joe Smith <joe.smith@foo.org>
  • If the patch has been submitted to LKML, but not accepted into any maintainer tree
    • tag the patch subject with FROMLIST:
    • add a Link: tag with a link to the submittal on lore.kernel.org
    • if changes were required, use BACKPORT: FROMLIST:
    • Example:
        FROMLIST: important patch from upstream

        This is the detailed description of the important patch

        Signed-off-by: Fred Jones <fred.jones@foo.org>

        Bug: 135791357
        Link: https://lore.kernel.org/lkml/20190619171517.GA17557@someone.com/
        Change-Id: I4caaaa566ea080fa148c5e768bb1a0b6f7201c01
        Signed-off-by: Joe Smith <joe.smith@foo.org>

Requirements for Android-specific patches: ANDROID:

  • If the patch is fixing a bug to Android-specific code
    • tag the patch subject with ANDROID:
    • add a Fixes: tag that cites the patch with the bug
    • Example:
        ANDROID: fix android-specific bug in foobar.c

        This is the detailed description of the important fix

        Fixes: 1234abcd2468 ("foobar: add cool feature")
        Change-Id: I4caaaa566ea080fa148c5e768bb1a0b6f7201c01
        Signed-off-by: Joe Smith <joe.smith@foo.org>
  • If the patch is a new feature
    • tag the patch subject with ANDROID:
    • add a Bug: tag with the Android bug (required for android-specific features)

Vibrator driver for HHG device

How to merge the driver into kernel source tree

  1. Copy ${this_project}/drivers/hid/hid-aksys.c into ${your_kernel_root}/drivers/hid/

  2. Compare and merge ${this_project}/drivers/hid/hid-ids.h into ${your_kernel_root}/drivers/hid/hid-ids.h : Add the following code before the last line of this file

    	#define USB_VENDER_ID_QUALCOMM  0x0a12
	#define USB_VENDER_ID_TEMP_HHG_AKSY 0x1234
	#define USB_PRODUCT_ID_AKSYS_HHG  0x1000

  3. Merge ${this_project}/drivers/hid/Kconfig into ${your_kernel_root}/drivers/hid/Kconfig : Add the following code before the last line of this file

    config HID_AKSYS_QRD
	tristate "AKSys gamepad USB adapter support"
	depends on HID
	---help---
	Support for AKSys gamepad USB adapter

config AKSYS_QRD_FF
	bool "AKSys gamepad USB adapter force feedback support"
	depends on HID_AKSYS_QRD
	select INPUT_FF_MEMLESS
	---help---
	Say Y here if you have a AKSys gamepad USB adapter and want to
	enable force feedback support for it.

  4. Merge ${this_project}/drivers/hid/Makefile into ${your_kernel_root}/drivers/hid/Makefile : Add the following code at the end of this file

    obj-$(CONFIG_HID_AKSYS_QRD)	+= hid-aksys.o

  5. Modify your kernel's default build configuration file. Add the following two lines:

    CONFIG_HID_AKSYS_QRD=m
CONFIG_AKSYS_QRD_FF=y