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udf: udf_sb.h: Replace zero-length array with flexible-array member
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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]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] KSPP#21
[3] commit 7649773 ("cxgb3/l2t: Fix undefined behaviour")

Link: https://lore.kernel.org/r/20200309202715.GA9428@embeddedor
Signed-off-by: Gustavo A. R. Silva <[email protected]>
Signed-off-by: Jan Kara <[email protected]>
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GustavoARSilva authored and jankara committed Mar 16, 2020
1 parent c2d0699 commit 3fc1316
Showing 1 changed file with 1 addition and 1 deletion.
2 changes: 1 addition & 1 deletion fs/udf/udf_sb.h
Original file line number Diff line number Diff line change
Expand Up @@ -83,7 +83,7 @@ struct udf_virtual_data {
struct udf_bitmap {
__u32 s_extPosition;
int s_nr_groups;
struct buffer_head *s_block_bitmap[0];
struct buffer_head *s_block_bitmap[];
};

struct udf_part_map {
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