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Using memdisk with grub2 and a compressed ISO

From: David Shaw
Subject: Using memdisk with grub2 and a compressed ISO
Date: Tue, 21 Apr 2015 18:29:51 -0400


I'm having a problem booting a compressed ISO image using memdisk via grub2 
(version 2.02, from CentOS 7).  The entry in my grub.cfg looks like:

menuentry 'boot ISO image' {
   linux16 /memdisk iso
   initrd16 /my-image.iso.gz

When selected, this starts to boot and then fails with:

  Ramdisk at 0x37979000, length 0x0033b298
  gzip image: decompressed addr 0x7f7f7000, len 0x00008f58: failed
  Decompression error: output buffer overrun

FWIW, my-image.iso.gz is 3388055 bytes long compressed, and 9394176 bytes long 
uncompressed.  The box in question has 8GiB of RAM.

Note that the uncompressed size of the iso in hex comes out to 8F5800.  The 
length given in the error message is 8F58 - that is, exactly 256 times too 
small.  That seems suspicious.

1) I have tried this with memdisk from syslinux 4.02, 4.05, and 6.03.  Same 
failure each time.
2) Changing the level of compression (i.e. gzip -1 instead of gzip -9) does not 
make it work.
3) Doing the same thing with grub1 (version 0.97), but using the same image and 
same version of memdisk, *does* work.
4) Uncompressing the ISO *does* make it work.
5) Using "zip" instead of "gzip" *does* make it work.

I've mailed with the syslinux / memdisk folks and they suggested coming here, 
as it looked like a grub2 issue.

Any suggestions on where to look next?



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