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Christophe Leroy fe534d76d4 powerpc/bitops: Fix possible undefined behaviour with fls() and fls64() 
commit 1891ef21d9 upstream.

fls() and fls64() are using __builtin_ctz() and _builtin_ctzll().
On powerpc, those builtins trivially use ctlzw and ctlzd power
instructions.

Allthough those instructions provide the expected result with
input argument 0, __builtin_ctz() and __builtin_ctzll() are
documented as undefined for value 0.

The easiest fix would be to use fls() and fls64() functions
defined in include/asm-generic/bitops/builtin-fls.h and
include/asm-generic/bitops/fls64.h, but GCC output is not optimal:

00000388 <testfls>:
 388:   2c 03 00 00     cmpwi   r3,0
 38c:   41 82 00 10     beq     39c <testfls+0x14>
 390:   7c 63 00 34     cntlzw  r3,r3
 394:   20 63 00 20     subfic  r3,r3,32
 398:   4e 80 00 20     blr
 39c:   38 60 00 00     li      r3,0
 3a0:   4e 80 00 20     blr

000003b0 <testfls64>:
 3b0:   2c 03 00 00     cmpwi   r3,0
 3b4:   40 82 00 1c     bne     3d0 <testfls64+0x20>
 3b8:   2f 84 00 00     cmpwi   cr7,r4,0
 3bc:   38 60 00 00     li      r3,0
 3c0:   4d 9e 00 20     beqlr   cr7
 3c4:   7c 83 00 34     cntlzw  r3,r4
 3c8:   20 63 00 20     subfic  r3,r3,32
 3cc:   4e 80 00 20     blr
 3d0:   7c 63 00 34     cntlzw  r3,r3
 3d4:   20 63 00 40     subfic  r3,r3,64
 3d8:   4e 80 00 20     blr

When the input of fls(x) is a constant, just check x for nullity and
return either 0 or __builtin_clz(x). Otherwise, use cntlzw instruction
directly.

For fls64() on PPC64, do the same but with __builtin_clzll() and
cntlzd instruction. On PPC32, lets take the generic fls64() which
will use our fls(). The result is as expected:

00000388 <testfls>:
 388:   7c 63 00 34     cntlzw  r3,r3
 38c:   20 63 00 20     subfic  r3,r3,32
 390:   4e 80 00 20     blr

000003a0 <testfls64>:
 3a0:   2c 03 00 00     cmpwi   r3,0
 3a4:   40 82 00 10     bne     3b4 <testfls64+0x14>
 3a8:   7c 83 00 34     cntlzw  r3,r4
 3ac:   20 63 00 20     subfic  r3,r3,32
 3b0:   4e 80 00 20     blr
 3b4:   7c 63 00 34     cntlzw  r3,r3
 3b8:   20 63 00 40     subfic  r3,r3,64
 3bc:   4e 80 00 20     blr

Fixes: 2fcff790dc ("powerpc: Use builtin functions for fls()/__fls()/fls64()")
Cc: stable@vger.kernel.org
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/348c2d3f19ffcff8abe50d52513f989c4581d000.1603375524.git.christophe.leroy@csgroup.eu
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
History 2020-12-30 11:54:16 +01:00
arch powerpc/bitops: Fix possible undefined behaviour with fls() and fls64() 2020-12-30 11:54:16 +01:00
block block-5.10-2020-12-05 2020-12-05 14:45:30 -08:00
certs .gitignore: add SPDX License Identifier 2020-03-25 11:50:48 +01:00
crypto crypto: ecdh - avoid unaligned accesses in ecdh_set_secret() 2020-12-30 11:54:10 +01:00
Documentation Documentation: seqlock: s/LOCKTYPE/LOCKNAME/g 2020-12-30 11:54:11 +01:00
drivers iommu/arm-smmu-qcom: Implement S2CR quirk 2020-12-30 11:54:15 +01:00
fs ext4: don't remount read-only with errors=continue on reboot 2020-12-30 11:54:13 +01:00
include binder: add flag to clear buffer on txn complete 2020-12-30 11:54:09 +01:00
init initramfs: fix clang build failure 2020-12-11 14:02:14 -08:00
ipc ipc: adjust proc_ipc_sem_dointvec definition to match prototype 2020-09-05 12:14:29 -07:00
kernel cpuset: fix race between hotplug work and later CPU offline 2020-12-30 11:54:11 +01:00
lib dyndbg: fix use before null check 2020-12-30 11:54:11 +01:00
LICENSES LICENSES/deprecated: add Zlib license text 2020-09-16 14:33:49 +02:00
mm z3fold: stricter locking and more careful reclaim 2020-12-30 11:54:10 +01:00
net xprtrdma: Fix XDRBUF_SPARSE_PAGES support 2020-12-30 11:54:15 +01:00
samples samples/bpf: Fix possible hang in xdpsock with multiple threads 2020-12-30 11:53:49 +01:00
scripts kconfig: fix return value of do_error_if() 2020-12-30 11:54:01 +01:00
security Smack: Handle io_uring kernel thread privileges 2020-12-30 11:54:02 +01:00
sound ASoC: AMD Raven/Renoir - fix the PCI probe (PCI revision) 2020-12-30 11:54:09 +01:00
tools perf probe: Fix memory leak when synthesizing SDT probes 2020-12-30 11:54:02 +01:00
usr Merge branch 'work.fdpic' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2020-08-07 13:29:39 -07:00
virt kvm: x86/mmu: Support dirty logging for the TDP MMU 2020-10-23 03:42:13 -04:00
.clang-format RDMA 5.10 pull request 2020-10-17 11:18:18 -07:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore .gitignore: docs: ignore sphinx_*/ directories 2020-09-10 10:44:31 -06:00
.mailmap mailmap: add two more addresses of Uwe Kleine-König 2020-12-06 10:19:07 -08:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Move Jason Cooper to CREDITS 2020-11-30 10:20:34 +01:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2020-12-10 15:30:13 -08:00
Makefile Linux 5.10.3 2020-12-26 16:02:46 +01:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -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 Restructured Text 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.