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From: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
To: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	"Gustavo A. R. Silva" <gustavo@embeddedor.com>
Subject: [PATCH] fs: select: Use struct_size() in kmalloc()
Date: Tue, 4 Jun 2019 11:42:26 -0500
Message-ID: <20190604164226.GA13823@embeddedor> (raw)

One of the more common cases of allocation size calculations is finding
the size of a structure that has a zero-sized array at the end, along
with memory for some number of elements for that array. For example:

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

size = sizeof(struct foo) + count * sizeof(struct boo);
instance = kmalloc(size, GFP_KERNEL);

Instead of leaving these open-coded and prone to type mistakes, we can
now use the new struct_size() helper:

instance = kmalloc(struct_size(instance, entry, count), GFP_KERNEL);

Also, notice that variable size is unnecessary, hence it is removed.

This code was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
---
 fs/select.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/select.c b/fs/select.c
index 6cbc9ff56ba0..2a179ab68c60 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -966,7 +966,7 @@ static int do_sys_poll(struct pollfd __user *ufds, unsigned int nfds,
 		struct timespec64 *end_time)
 {
 	struct poll_wqueues table;
- 	int err = -EFAULT, fdcount, len, size;
+	int err = -EFAULT, fdcount, len;
 	/* Allocate small arguments on the stack to save memory and be
 	   faster - use long to make sure the buffer is aligned properly
 	   on 64 bit archs to avoid unaligned access */
@@ -994,8 +994,8 @@ static int do_sys_poll(struct pollfd __user *ufds, unsigned int nfds,
 			break;
 
 		len = min(todo, POLLFD_PER_PAGE);
-		size = sizeof(struct poll_list) + sizeof(struct pollfd) * len;
-		walk = walk->next = kmalloc(size, GFP_KERNEL);
+		walk = walk->next = kmalloc(struct_size(walk, entries, len),
+					    GFP_KERNEL);
 		if (!walk) {
 			err = -ENOMEM;
 			goto out_fds;
-- 
2.21.0


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