libkmod: Fix file error handling regression

[stoeckmann]

The propagation of errors from loading function through errno must make sure that errno is positive, otherwise errors are not correctly spotted.

Proof of Concept:

1. Create invalid module files of different file types

base64 -d <<< AAAAAAAAAA== > poc.ko
base64 -d <<< H4sAAAAAAA== > poc.ko.gz
base64 -d <<< /Td6WFoAAA== > poc.ko.xz
base64 -d <<< KLUv/QAAAA== > poc.ko.zst

2. Run modinfo and check exit code

for i in poc.ko*
do
        echo "$i:"
        modinfo $i
        echo $?
done

The regression occurs in kmod 33 already. Its output is:

poc.ko:
filename:       /tmp/poc.ko
modinfo: ERROR: could not get modinfo from 'poc': Exec format error
1
poc.ko.gz:
filename:       /tmp/poc.ko.gz
libkmod: kmod_file_load_zlib: gzip: <fd:4>: unknown compression method
0
poc.ko.xz:
filename:       /tmp/poc.ko.xz
libkmod: xz_uncompress_belch: xz: Unexpected end of input
0
poc.ko.zst:
filename:       /tmp/poc.ko.zst
modinfo: ERROR: could not get modinfo from 'poc': Exec format error
1

With current master, we see:

poc.ko:
filename:       /tmp/poc.ko
modinfo: ERROR: could not get modinfo from 'poc': Exec format error
1
poc.ko.gz:
filename:       /tmp/poc.ko.gz
libkmod: ERROR: kmod_file_load_zlib: gzip: <fd:4>: unknown compression method
0
poc.ko.xz:
filename:       /tmp/poc.ko.xz
libkmod: ERROR: xz_uncompress_belch: xz: Unexpected end of input
0
poc.ko.zst:
filename:       /tmp/poc.ko.zst
libkmod: ERROR: kmod_file_load_zstd: zstd: Failed to determine decompression size
0

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
libkmod/libkmod-file-zstd.c	0.00%	1 Missing ⚠️
libkmod/libkmod-file.c	0.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
libkmod/libkmod-file-zstd.c	52.50% <0.00%> (ø)
libkmod/libkmod-file.c	71.01% <0.00%> (ø)

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[evelikov]

Not sure if I call this a regression - nevertheless, nicely spotted.

[lucasdemarchi]

Applied, thanks