Re: [PATCH for-5.10.y] lib/lz4: explicitly support in-place decompression

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, Jun 01, 2021 at 11:46:45PM +0800, Gao Xiang wrote:
> 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.

Both now queued up, thanks!

greg k-h