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commit382d4cd184upstream. The gcc compiler translates on some architectures the 64-bit __builtin_clzll() function to a call to the libgcc function __clzdi2(), which should take a 64-bit parameter on 32- and 64-bit platforms. But in the current kernel code, the built-in __clzdi2() function is defined to operate (wrongly) on 32-bit parameters if BITS_PER_LONG == 32, thus the return values on 32-bit kernels are in the range from [0..31] instead of the expected [0..63] range. This patch fixes the in-kernel functions __clzdi2() and __ctzdi2() to take a 64-bit parameter on 32-bit kernels as well, thus it makes the functions identical for 32- and 64-bit kernels. This bug went unnoticed since kernel 3.11 for over 10 years, and here are some possible reasons for that: a) Some architectures have assembly instructions to count the bits and which are used instead of calling __clzdi2(), e.g. on x86 the bsr instruction and on ppc cntlz is used. On such architectures the wrong __clzdi2() implementation isn't used and as such the bug has no effect and won't be noticed. b) Some architectures link to libgcc.a, and the in-kernel weak functions get replaced by the correct 64-bit variants from libgcc.a. c) __builtin_clzll() and __clzdi2() doesn't seem to be used in many places in the kernel, and most likely only in uncritical functions, e.g. when printing hex values via seq_put_hex_ll(). The wrong return value will still print the correct number, but just in a wrong formatting (e.g. with too many leading zeroes). d) 32-bit kernels aren't used that much any longer, so they are less tested. A trivial testcase to verify if the currently running 32-bit kernel is affected by the bug is to look at the output of /proc/self/maps: Here the kernel uses a correct implementation of __clzdi2(): root@debian:~# cat /proc/self/maps 00010000-00019000 r-xp 00000000 08:05 787324 /usr/bin/cat 00019000-0001a000 rwxp 00009000 08:05 787324 /usr/bin/cat 0001a000-0003b000 rwxp 00000000 00:00 0 [heap] f7551000-f770d000 r-xp 00000000 08:05 794765 /usr/lib/hppa-linux-gnu/libc.so.6 ... and this kernel uses the broken implementation of __clzdi2(): root@debian:~# cat /proc/self/maps 0000000010000-0000000019000 r-xp 00000000 000000008:000000005 787324 /usr/bin/cat 0000000019000-000000001a000 rwxp 000000009000 000000008:000000005 787324 /usr/bin/cat 000000001a000-000000003b000 rwxp 00000000 00:00 0 [heap] 00000000f73d1000-00000000f758d000 r-xp 00000000 000000008:000000005 794765 /usr/lib/hppa-linux-gnu/libc.so.6 ... Signed-off-by: Helge Deller <deller@gmx.de> Fixes:4df87bb7b6("lib: add weak clz/ctz functions") Cc: Chanho Min <chanho.min@lge.com> Cc: Geert Uytterhoeven <geert@linux-m68k.org> Cc: stable@vger.kernel.org # v3.11+ Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
44 lines
899 B
C
44 lines
899 B
C
// SPDX-License-Identifier: GPL-2.0-only
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/*
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* lib/clz_ctz.c
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*
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* Copyright (C) 2013 Chanho Min <chanho.min@lge.com>
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*
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* The functions in this file aren't called directly, but are required by
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* GCC builtins such as __builtin_ctz, and therefore they can't be removed
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* despite appearing unreferenced in kernel source.
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*
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* __c[lt]z[sd]i2 can be overridden by linking arch-specific versions.
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*/
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#include <linux/export.h>
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#include <linux/kernel.h>
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int __weak __ctzsi2(int val);
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int __weak __ctzsi2(int val)
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{
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return __ffs(val);
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}
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EXPORT_SYMBOL(__ctzsi2);
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int __weak __clzsi2(int val);
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int __weak __clzsi2(int val)
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{
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return 32 - fls(val);
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}
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EXPORT_SYMBOL(__clzsi2);
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int __weak __clzdi2(u64 val);
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int __weak __clzdi2(u64 val)
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{
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return 64 - fls64(val);
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}
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EXPORT_SYMBOL(__clzdi2);
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int __weak __ctzdi2(u64 val);
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int __weak __ctzdi2(u64 val)
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{
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return __ffs64(val);
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}
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EXPORT_SYMBOL(__ctzdi2);
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