grub may hang or ignore on bad/modified *.gz

[Jan Beulich]

As is common practice with (at least) SuSE distributions, and as is also 
required by the ACPI DSDT override mechanism, a Linux initrd may contain more 
information than just the gzipped file system image. Since grub only looks at 
the last 8 bytes to determine crc and original size, it must be very careful 
when interpreting these or attempting to unzip such a file. Currently what 
happens is that

- if the original size happens to be zero or negative (as it is treated as a 
signed value), the entire file is ignored (cmp or module simply do nothing)
- if the original size happens to be positive, good chances are that grub 
actually hangs appearantly due to missing validity checks in the unzipping code

For the first part, detecting a non-positive original size in 
gunzip_test_header() should help.

For the second part, since gzip detects and reports the presence of additional 
data past the compressed part, perhaps similar logic should be used here. 
Ideally, the read would then be failed in a way that the original caller can 
revert to treat the file as not needing decompression, but in no case should 
the partially handled file be passed as a module to the kernel or be silently 
ignored.

Thanks, Jan