6.4-stable review patch. If anyone has any objections, please let me know.
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From: Helge Deller deller@gmx.de
commit 382d4cd1847517ffcb1800fd462b625db7b2ebea upstream.
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: 4df87bb7b6a22 ("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 --- lib/clz_ctz.c | 32 ++++++-------------------------- 1 file changed, 6 insertions(+), 26 deletions(-)
--- a/lib/clz_ctz.c +++ b/lib/clz_ctz.c @@ -28,36 +28,16 @@ int __weak __clzsi2(int val) } EXPORT_SYMBOL(__clzsi2);
-int __weak __clzdi2(long val); -int __weak __ctzdi2(long val); -#if BITS_PER_LONG == 32 - -int __weak __clzdi2(long val) +int __weak __clzdi2(u64 val); +int __weak __clzdi2(u64 val) { - return 32 - fls((int)val); + return 64 - fls64(val); } EXPORT_SYMBOL(__clzdi2);
-int __weak __ctzdi2(long val) +int __weak __ctzdi2(u64 val); +int __weak __ctzdi2(u64 val) { - return __ffs((u32)val); + return __ffs64(val); } EXPORT_SYMBOL(__ctzdi2); - -#elif BITS_PER_LONG == 64 - -int __weak __clzdi2(long val) -{ - return 64 - fls64((u64)val); -} -EXPORT_SYMBOL(__clzdi2); - -int __weak __ctzdi2(long val) -{ - return __ffs64((u64)val); -} -EXPORT_SYMBOL(__ctzdi2); - -#else -#error BITS_PER_LONG not 32 or 64 -#endif