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If /proc/config.gz is unavailable, how do I know what decompression algorithms the running kernel is capable of using on a compressed cpio initramfs?

Is the gzip algorithm always available, even when CONFIG_DECOMPRESS_GZIP is not y when building the kernel?

2

Compression algorithms are declared in lib/decompress.c. Gzip is defined in lib/decompress_inflate.c and doesn't get any special status; it'll only be there if CONFIG_DECOMPRESS_GZIP is y when the kernel is compiled.

The list of available compression algorithms is the compressed_formats structure. Since it's defined as static, it isn't available in other source files. The only function that uses it is thus the decompress_method function in lib/decompress.c itself. So the only way to get a kernel to use the table of supported algorithms is to attempt to decompress something that starts with the appropriate two-byte magic sequence, and see if that function returns the algorithm name.

You can tell which decompression functions are supported by searching the list of kernel symbols (/proc/kallsyms). The functions aren't identified as such, so you'll need to bake in the list of symbol names.

</proc/kallsyms cut -d " " -f 3 |
grep -xF -e gunzip -e bzip2 -e unlzma -e unxz -e unlzo -e unlz4
0

You can get to know your compression type by going into the directory sys/power/tuxonice/compression there u will a file named algorithm go into it it will have the decompression type in it for me it was written lzo.

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