From: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Subject: pstore/ram: Replace zero-length array with flexible-array member
Git-commit: 8128d3aac0ee3420ede34950c9c0ef9ee118bec9
Patch-mainline: v5.7
References: jsc#SLE-16304
Acked-by: Jiri Bohac <jbohac@suse.cz>

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://lore.kernel.org/r/20200309202327.GA8813@embeddedor
Signed-off-by: Kees Cook <keescook@chromium.org>

diff --git a/fs/pstore/ram_core.c b/fs/pstore/ram_core.c
index 1f4d8c06f9be..c917c191e78c 100644
--- a/fs/pstore/ram_core.c
+++ b/fs/pstore/ram_core.c
@@ -34,7 +34,7 @@ struct persistent_ram_buffer {
 	uint32_t    sig;
 	atomic_t    start;
 	atomic_t    size;
-	uint8_t     data[0];
+	uint8_t     data[];
 };
 
 #define PERSISTENT_RAM_SIG (0x43474244) /* DBGC */
-- 
2.28.0