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
Cc: Yann Collet <[email protected]>
Cc: Nick Terrell <[email protected]>
Cc: Miao Xie <[email protected]>
Cc: Chao Yu <[email protected]>
Cc: Li Guifu <[email protected]>
Cc: Guo Xuenan <[email protected]>
Signed-off-by: Gao Xiang <[email protected]>
---
changes since v1:
- refine commit message;
- Cc more people.
Hi Andrew,
Could you kindly consider picking this patch up, although
the impact is EROFS but it touchs in-kernel lz4 library
anyway...
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)
{
--
2.18.4
