* [PATCH] apparmor: Replace zero-length array with flexible-array
@ 2020-05-07 18:43 Gustavo A. R. Silva
2020-05-15 20:03 ` John Johansen
0 siblings, 1 reply; 3+ messages in thread
From: Gustavo A. R. Silva @ 2020-05-07 18:43 UTC (permalink / raw)
To: John Johansen; +Cc: linux-kernel
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]
sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.
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 <gustavoars@kernel.org>
---
security/apparmor/apparmorfs.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/security/apparmor/apparmorfs.c b/security/apparmor/apparmorfs.c
index 280741fc0f5f..299994ff9168 100644
--- a/security/apparmor/apparmorfs.c
+++ b/security/apparmor/apparmorfs.c
@@ -839,7 +839,7 @@ static ssize_t query_label(char *buf, size_t buf_len,
struct multi_transaction {
struct kref count;
ssize_t size;
- char data[0];
+ char data[];
};
#define MULTI_TRANSACTION_LIMIT (PAGE_SIZE - sizeof(struct multi_transaction))
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] apparmor: Replace zero-length array with flexible-array
2020-05-07 18:43 [PATCH] apparmor: Replace zero-length array with flexible-array Gustavo A. R. Silva
@ 2020-05-15 20:03 ` John Johansen
2020-05-15 20:13 ` Gustavo A. R. Silva
0 siblings, 1 reply; 3+ messages in thread
From: John Johansen @ 2020-05-15 20:03 UTC (permalink / raw)
To: Gustavo A. R. Silva; +Cc: linux-kernel
On 5/7/20 11:43 AM, Gustavo A. R. Silva wrote:
> 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]
>
> sizeof(flexible-array-member) triggers a warning because flexible array
> members have incomplete type[1]. There are some instances of code in
> which the sizeof operator is being incorrectly/erroneously applied to
> zero-length arrays and the result is zero. Such instances may be hiding
> some bugs. So, this work (flexible-array member conversions) will also
> help to get completely rid of those sorts of issues.
>
> 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 <gustavoars@kernel.org>
Acked-by: John Johansen <john.johansen@canonical.com>
I have pulled this into my tree
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] apparmor: Replace zero-length array with flexible-array
2020-05-15 20:03 ` John Johansen
@ 2020-05-15 20:13 ` Gustavo A. R. Silva
0 siblings, 0 replies; 3+ messages in thread
From: Gustavo A. R. Silva @ 2020-05-15 20:13 UTC (permalink / raw)
To: John Johansen; +Cc: linux-kernel
On Fri, May 15, 2020 at 01:03:47PM -0700, John Johansen wrote:
> >
> > Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> Acked-by: John Johansen <john.johansen@canonical.com>
>
> I have pulled this into my tree
Great. :)
Thanks, John.
--
Gustavo
^ permalink raw reply [flat|nested] 3+ messages in thread
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2020-05-07 18:43 [PATCH] apparmor: Replace zero-length array with flexible-array Gustavo A. R. Silva
2020-05-15 20:03 ` John Johansen
2020-05-15 20:13 ` Gustavo A. R. Silva
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