[PATCH] lib/decompress_unlz4.c: correctly handle zero-padding around initrds.

[Dimitri John Ledkov]

lz4 compatible decompressor is simple. The format is underspecified and
relies on EOF notification to determine when to stop. Initramfs buffer
format[1] explicitly states that it can have arbitrary number of zero
padding. Thus when operating without a fill function, be extra careful to
ensure that sizes less than 4, or apperantly empty chunksizes are treated
as EOF.

To test this I have created two cpio initrds, first a normal one,
main.cpio. And second one with just a single /test-file with content
"second" second.cpio. Then i compressed both of them with gzip, and with
lz4 -l. Then I created a padding of 4 bytes (dd if=/dev/zero of=pad4 bs=1
count=4). To create four testcase initrds:

 1) main.cpio.gzip + extra.cpio.gzip = pad0.gzip
 2) main.cpio.lz4  + extra.cpio.lz4 = pad0.lz4
 3) main.cpio.gzip + pad4 + extra.cpio.gzip = pad4.gzip
 4) main.cpio.lz4  + pad4 + extra.cpio.lz4 = pad4.lz4

The pad4 test-cases replicate the initrd load by grub, as it pads and
aligns every initrd it loads.

All of the above boot, however /test-file was not accessible in the initrd
for the testcase #4, as decoding in lz4 decompressor failed. Also an error
message printed which usually is harmless.

Whith a patched kernel, all of the above testcases now pass, and /test-file
is accessible.

This fixes lz4 initrd decompress warning on every boot with grub. And more
importantly this fixes inability to load multiple lz4 compressed initrds
with grub.

I guess I should convert above decompressor streams with/without padding
into kunit tests, across all decompressor algorithms.

[1] ./Documentation/driver-api/early-userspace/buffer-format.rst

BugLink: https://bugs.launchpad.net/bugs/1835660
Signed-off-by: Dimitri John Ledkov <xnox@xxxxxxxxxx>
---
 lib/decompress_unlz4.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/lib/decompress_unlz4.c b/lib/decompress_unlz4.c
index c0cfcfd486be..e6327391b6b6 100644
--- a/lib/decompress_unlz4.c
+++ b/lib/decompress_unlz4.c
@@ -112,6 +112,9 @@ STATIC inline int INIT unlz4(u8 *input, long in_len,
 				error("data corrupted");
 				goto exit_2;
 			}
+		} else if (size < 4) {
+			/* empty or end-of-file */
+			goto exit_3;
 		}
 
 		chunksize = get_unaligned_le32(inp);
@@ -125,6 +128,10 @@ STATIC inline int INIT unlz4(u8 *input, long in_len,
 			continue;
 		}
 
+		if (!fill && chunksize == 0) {
+			/* empty or end-of-file */
+			goto exit_3;
+		}
 
 		if (posp)
 			*posp += 4;
@@ -184,6 +191,7 @@ STATIC inline int INIT unlz4(u8 *input, long in_len,
 		}
 	}
 
+exit_3:
 	ret = 0;
 exit_2:
 	if (!input)
-- 
2.27.0