commit 89b158635ad79574bde8e94d45dad33f8cf09549 upstream.
LZ4 final literal copy could be overlapped when doing in-place decompression, so it's unsafe to just use memcpy() on an optimized memcpy approach but memmove() instead.
Upstream LZ4 has updated this years ago [1] (and the impact is non-sensible [2] plus only a few bytes remain), this commit just synchronizes LZ4 upstream code to the kernel side as well.
It can be observed as EROFS in-place decompression failure on specific files when X86_FEATURE_ERMS is unsupported, memcpy() optimization of commit 59daa706fbec ("x86, mem: Optimize memcpy by avoiding memory false dependece") will be enabled then.
Currently most modern x86-CPUs support ERMS, these CPUs just use "rep movsb" approach so no problem at all. However, it can still be verified with forcely disabling ERMS feature...
arch/x86/lib/memcpy_64.S: ALTERNATIVE_2 "jmp memcpy_orig", "", X86_FEATURE_REP_GOOD, \ - "jmp memcpy_erms", X86_FEATURE_ERMS + "jmp memcpy_orig", X86_FEATURE_ERMS
We didn't observe any strange on arm64/arm/x86 platform before since most memcpy() would behave in an increasing address order ("copy upwards" [3]) and it's the correct order of in-place decompression but it really needs an update to memmove() for sure considering it's an undefined behavior according to the standard and some unique optimization already exists in the kernel.
[1] https://github.com/lz4/lz4/commit/33cb8518ac385835cc17be9a770b27b40cd0e15b [2] https://github.com/lz4/lz4/pull/717#issuecomment-497818921 [3] https://sourceware.org/bugzilla/show_bug.cgi?id=12518
Link: https://lkml.kernel.org/r/20201122030749.2698994-1-hsiangkao@redhat.com Reviewed-by: Nick Terrell terrelln@fb.com Cc: Yann Collet yann.collet.73@gmail.com Cc: Miao Xie miaoxie@huawei.com Cc: Chao Yu yuchao0@huawei.com Cc: Li Guifu bluce.liguifu@huawei.com Cc: Guo Xuenan guoxuenan@huawei.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Gao Xiang hsiangkao@linux.alibaba.com --- Hi,
Please kindly consider these two backports to 5.4.y and 5.10.y LTS kernels, and the reason shown as above (it could cause lz4 in-place decompression (mainly EROFS) failure due to the different designed memcpy overlapped behavior on x86 if ERMS is unsupported.) The lz4 upstream commit itself has been merged for 2 years. And the linux upstream commit is also merged for months without any other regression.
And in principle, it won't have any real impact at all, so I think it's now safe to backport this to LTS kernels for unsupported ERMS x86s.
Thanks, Gao Xiang
lib/lz4/lz4_decompress.c | 6 +++++- lib/lz4/lz4defs.h | 1 + 2 files changed, 6 insertions(+), 1 deletion(-)
diff --git a/lib/lz4/lz4_decompress.c b/lib/lz4/lz4_decompress.c index 00cb0d0b73e1..8a7724a6ce2f 100644 --- a/lib/lz4/lz4_decompress.c +++ b/lib/lz4/lz4_decompress.c @@ -263,7 +263,11 @@ static FORCE_INLINE int LZ4_decompress_generic( } }
- LZ4_memcpy(op, ip, length); + /* + * supports overlapping memory regions; only matters + * for in-place decompression scenarios + */ + LZ4_memmove(op, ip, length); ip += length; op += length;
diff --git a/lib/lz4/lz4defs.h b/lib/lz4/lz4defs.h index c91dd96ef629..673bd206aa98 100644 --- a/lib/lz4/lz4defs.h +++ b/lib/lz4/lz4defs.h @@ -146,6 +146,7 @@ static FORCE_INLINE void LZ4_writeLE16(void *memPtr, U16 value) * environments. This is needed when decompressing the Linux Kernel, for example. */ #define LZ4_memcpy(dst, src, size) __builtin_memcpy(dst, src, size) +#define LZ4_memmove(dst, src, size) __builtin_memmove(dst, src, size)
static FORCE_INLINE void LZ4_copy8(void *dst, const void *src) {