bootm: improve error message when gzip decompression buffer is too small

Currently, when decompressing a gzip-compressed image during bootm, a
generic error such as "inflate() returned -5" is shown when the buffer is
too small. However, it is not immediately clear that this is caused by
CONFIG_SYS_BOOTM_LEN being too small.

This patch improves error handling by:
- Detecting Z_BUF_ERROR (-5) returned from the inflate() call
- Suggesting the user to increase CONFIG_SYS_BOOTM_LEN when applicable
- Preserving the original return code from zunzip() instead of overwriting
  it with -1

By providing clearer hints when decompression fails due to insufficient
buffer size, this change helps users diagnose and fix boot failures more
easily.

Signed-off-by: Aristo Chen <aristo.chen@canonical.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@kernel.org>

boot/bootm.c

@@ -34,6 +34,7 @@
 
 #include <bootm.h>
 #include <image.h>
+#include <u-boot/zlib.h>
 
 #define MAX_CMDLINE_SIZE	SZ_4K
 
@@ -578,7 +579,8 @@ static int handle_decomp_error(int comp_type, size_t uncomp_size,
 	if (ret == -ENOSYS)
 		return BOOTM_ERR_UNIMPLEMENTED;
 
-	if (uncomp_size >= buf_size)
+	if ((comp_type == IH_COMP_GZIP && ret == Z_BUF_ERROR) ||
+	    uncomp_size >= buf_size)
 		printf("Image too large: increase CONFIG_SYS_BOOTM_LEN\n");
 	else
 		printf("%s: uncompress error %d\n", name, ret);

lib/gunzip.c

@@ -299,7 +299,7 @@ __rcode int zunzip(void *dst, int dstlen, unsigned char *src,
 		if (stoponerr == 1 && r != Z_STREAM_END &&
 		    (s.avail_in == 0 || s.avail_out == 0 || r != Z_BUF_ERROR)) {
 			printf("Error: inflate() returned %d\n", r);
-			err = -1;
+			err = r;
 			break;
 		}
 	} while (r == Z_BUF_ERROR);