Corrected edge case in uncompressed ARC->L2ARC handling

I genuinely don't know why this didn't come up before,
but adding the LZ4 early abort pointed out this flaw,
in which we're allocating a buffer of one size, and
then telling the compressor that we're handing it buffers
of a different size, which may be Very Different - say,
allocating 512b and then telling it the inputs are 128k.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Amanakis <gamanakis@gmail.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes openzfs#13375

module/zfs/arc.c

@@ -9337,26 +9337,37 @@ l2arc_apply_transforms(spa_t *spa, arc_buf_hdr_t *hdr, uint64_t asize,
 	}
 
 	if (compress != ZIO_COMPRESS_OFF && !HDR_COMPRESSION_ENABLED(hdr)) {
-		cabd = abd_alloc_for_io(asize, ismd);
-		tmp = abd_borrow_buf(cabd, asize);
+		/*
+		 * In some cases, we can wind up with size > asize, so
+		 * we need to opt for the larger allocation option here.
+		 *
+		 * (We also need abd_return_buf_copy in all cases because
+		 * it's an ASSERT() to modify the buffer before returning it
+		 * with arc_return_buf(), and all the compressors
+		 * write things before deciding to fail compression in nearly
+		 * every case.)
+		 */
+		cabd = abd_alloc_for_io(size, ismd);
+		tmp = abd_borrow_buf(cabd, size);
 
 		psize = zio_compress_data(compress, to_write, tmp, size,
 		    hdr->b_complevel);
 
-		if (psize >= size) {
-			abd_return_buf(cabd, tmp, asize);
+		if (psize >= asize) {
+			psize = HDR_GET_PSIZE(hdr);
+			abd_return_buf_copy(cabd, tmp, size);
 			HDR_SET_COMPRESS(hdr, ZIO_COMPRESS_OFF);
 			to_write = cabd;
-			abd_copy(to_write, hdr->b_l1hdr.b_pabd, size);
-			if (size != asize)
-				abd_zero_off(to_write, size, asize - size);
+			abd_copy(to_write, hdr->b_l1hdr.b_pabd, psize);
+			if (psize != asize)
+				abd_zero_off(to_write, psize, asize - psize);
 			goto encrypt;
 		}
 		ASSERT3U(psize, <=, HDR_GET_PSIZE(hdr));
 		if (psize < asize)
 			memset((char *)tmp + psize, 0, asize - psize);
 		psize = HDR_GET_PSIZE(hdr);
-		abd_return_buf_copy(cabd, tmp, asize);
+		abd_return_buf_copy(cabd, tmp, size);
 		to_write = cabd;
 	}