[PATCH master] ubifs: fix crash building without decompressors

[Ahmad Fatoum]

When building barebox with ubifs support, but without decompressors
enabled, we run into a NULL pointer dereference:

  BUG: failure at lib/string.c:249/strncmp()!
  BUG!
  [<4fd8ea59>] (unwind_backtrace+0x1/0x78) from [<4fd014e5>] (panic+0x1d/0x34)
  [<4fd014e5>] (panic+0x1d/0x34) from [<4fd65ed1>] (strncmp+0x2d/0x50)
  [<4fd65ed1>] (strncmp+0x2d/0x50) from [<4fd784bb>] (compr_init+0x4b/0x70)
  [<4fd784bb>] (compr_init+0x4b/0x70) from [<4fd78915>] (ubifs_compressors_init+0x15/0x40)
  [<4fd78915>] (ubifs_compressors_init+0x15/0x40) from [<4fd78e89>] (ubifs_init+0x2d/0x48)
  [<4fd78e89>] (ubifs_init+0x2d/0x48) from [<4fd01159>] (start_barebox+0x35/0x6c)
  [<4fd01159>] (start_barebox+0x35/0x6c) from [<4fd8c9f3>] (barebox_non_pbl_start+0x127/0x170)
  [<4fd8c9f3>] (barebox_non_pbl_start+0x127/0x170) from [<4fd00005>] (__bare_init_start+0x1/0xc)

This is because the loop in crypto_alloc_comp iterates over all possible
decompressors and then accesses comp->capi_name, which will be NULL for
unusable decompressors. Add a NULL check to handle this gracefully.
A check for alg_name isn't needed because it's already checked in
compr_init.

Signed-off-by: Ahmad Fatoum <a.fatoum at pengutronix.de>
---
 fs/ubifs/ubifs.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/ubifs/ubifs.c b/fs/ubifs/ubifs.c
index e7b434c1960e..0b4f3de773e7 100644
--- a/fs/ubifs/ubifs.c
+++ b/fs/ubifs/ubifs.c
@@ -125,7 +125,8 @@ static inline struct crypto_comp
 			i++;
 			continue;
 		}
-		if (strncmp(alg_name, comp->capi_name, strlen(alg_name)) == 0) {
+		if (comp->capi_name &&
+		    strncmp(alg_name, comp->capi_name, strlen(alg_name)) == 0) {
 			ptr->compressor = i;
 			return ptr;
 		}
-- 
2.30.2