linux-kernel - [PATCH][next] fs: select: Replace zero-length array with flexible-array member

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Message-ID: <20200309202449.GA9027@embeddedor>
Date:   Mon, 9 Mar 2020 15:24:49 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Alexander Viro <viro@...iv.linux.org.uk>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH][next] fs: select: Replace zero-length array with
 flexible-array member

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@...eddedor.com>
---
 fs/select.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/select.c b/fs/select.c
index 11d0285d46b7..f38a8a7480f7 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -97,7 +97,7 @@ u64 select_estimate_accuracy(struct timespec64 *tv)
 struct poll_table_page {
 	struct poll_table_page * next;
 	struct poll_table_entry * entry;
-	struct poll_table_entry entries[0];
+	struct poll_table_entry entries[];
 };
 
 #define POLL_TABLE_FULL(table) \
@@ -826,7 +826,7 @@ SYSCALL_DEFINE1(old_select, struct sel_arg_struct __user *, arg)
 struct poll_list {
 	struct poll_list *next;
 	int len;
-	struct pollfd entries[0];
+	struct pollfd entries[];
 };
 
 #define POLLFD_PER_PAGE  ((PAGE_SIZE-sizeof(struct poll_list)) / sizeof(struct pollfd))
-- 
2.25.0