Messages in this thread | | | From | Vineet Gupta <> | Subject | Heads up: gcc miscompiling initramfs zlib decompression code at -O3 | Date | Fri, 30 Apr 2021 20:46:16 +0000 |
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Hi,
I've hit a mainline gcc 10.2 (also gcc 9.3) bug which triggers at -O3 causing wrong codegen.
Config needs to have initramfs + gzip compressed.
CONFIG_HAVE_KERNEL_GZIP=y CONFIG_KERNEL_GZIP=y CONFIG_DECOMPRESS_GZIP=y CONFIG_INITRAMFS_COMPRESSION_GZIP=y
lib/zlib_inflate/inffast.c
if (dist > 2) { unsigned short *sfrom;
sfrom = (unsigned short *)(from); loops = len >> 1; do *sout++ = *sfrom++; ^^^^^^ ^^^^^^^^ while (--loops); out = (unsigned char *)sout; from = (unsigned char *)sfrom; } ...
The gist of issue is that despite use of unsigned short pointers, gcc is generating wider load/stores (8-byte LDD/STD on arcv2 and 16-byte on aarch64) causing extraneous bytes to copied into inflated gzip binaries manifesting later as corrupted fragments in the binaries.
I've opened a gcc bug at: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100363
The workaround is to build lib/zlib_inflate/inffast.c with -O2, although I reckon not many arches build with -O3 as default. I'll be proposing an ARC only patch to build this file with -O2, unless people think it needs to be generalized.
Also problem originally seen on 5.6 kernel, although I confirm it shows on latest mainline as well.
Unraveling this pretty fun, gory details for those interested at:
https://github.com/foss-for-synopsys-dwc-arc-processors/toolchain/issues/372
Thx, -Vineet
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