[PATCH v4 4/5] erofs-utils: zero out garbage trailing data for non-0padding cases

[Gao Xiang <xiang@kernel.org>]

When "-E legacy-compress" is used, lz4 0padding feature will be
disabled by default in order to support old kernels (< Linux v5.3).

In that case, the current mkfs leaves previous garbage data after
valid compressed data if the length becomes shorter. This doesn't
matter for kernels >= v5.0 since LZ4_decompress_safe_partial() is used.

However, for staging erofs v4.19, it used an in-house customized
lz4 implemention due to LZ4_decompress_safe_partial() didn't work
as expected at that time, yet it doesn't allow trailing random
data in practice or decompression failure could happen.

I don't think it really matters since "obsoleted_mkfs" works perfectly
for such old staging versions (v4.19). Anyway, trailing garbage data
sounds unreasonable, so let's zero out it now.

Fixes: 66653ef10a7f ("erofs-utils: introduce compression for regular files")
Signed-off-by: Gao Xiang <xiang@kernel.org>
---
 lib/compress.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/lib/compress.c b/lib/compress.c
index b8bb89e7ae9d..deef6a2c8ae3 100644
--- a/lib/compress.c
+++ b/lib/compress.c
@@ -189,18 +189,22 @@ nocompression:
 			ctx->compressedblks = 1;
 			raw = true;
 		} else {
-			const unsigned int used = ret & (EROFS_BLKSIZ - 1);
-			const unsigned int margin =
-				erofs_sb_has_lz4_0padding() && used ?
-					EROFS_BLKSIZ - used : 0;
+			const unsigned int tailused = ret & (EROFS_BLKSIZ - 1);
+			const unsigned int padding =
+				erofs_sb_has_lz4_0padding() && tailused ?
+					EROFS_BLKSIZ - tailused : 0;
 
 			ctx->compressedblks = DIV_ROUND_UP(ret, EROFS_BLKSIZ);
+			/* zero out garbage trailing data for non-0padding */
+			if (!erofs_sb_has_lz4_0padding())
+				memset(dst + ret, 0,
+				       roundup(ret, EROFS_BLKSIZ) - ret);
 
 			/* write compressed data */
 			erofs_dbg("Writing %u compressed data to %u of %u blocks",
 				  count, ctx->blkaddr, ctx->compressedblks);
 
-			ret = blk_write(dst - margin, ctx->blkaddr,
+			ret = blk_write(dst - padding, ctx->blkaddr,
 					ctx->compressedblks);
 			if (ret)
 				return ret;
-- 
2.20.1