Re: [PATCH] dmaengine: at_xdmac: Replace zero-length array with flexible-array
From: Ludovic Desroches <ludovic.desroches@microchip.com>
Date: 2020-05-11 05:48:16
Also in:
dmaengine, lkml
On Thu, May 07, 2020 at 02:00:46PM -0500, Gustavo A. R. Silva wrote:
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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: Ludovic Desroches<ludovic.desroches@microchip.com> Ludovic Desroches
quoted hunk ↗ jump to hunk
--- drivers/dma/at_xdmac.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)diff --git a/drivers/dma/at_xdmac.c b/drivers/dma/at_xdmac.c index bb0eaf38b594..fd92f048c491 100644 --- a/drivers/dma/at_xdmac.c +++ b/drivers/dma/at_xdmac.c@@ -212,7 +212,7 @@ struct at_xdmac { struct clk *clk; u32 save_gim; struct dma_pool *at_xdmac_desc_pool; - struct at_xdmac_chan chan[0]; + struct at_xdmac_chan chan[]; };
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