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ext4: use flexible-array member in struct fname
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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] https://github.com/KSPP/linux/issues/21
[3] commit 7649773 ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://lore.kernel.org/r/20200309154838.GA31559@embeddedor
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
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Gustavo A. R. Silva authored and Theodore Ts'o committed Mar 14, 2020
1 parent 7af44c4 commit e32ac24
Showing 1 changed file with 1 addition and 1 deletion.
2 changes: 1 addition & 1 deletion fs/ext4/dir.c
Original file line number Diff line number Diff line change
Expand Up @@ -392,7 +392,7 @@ struct fname {
__u32 inode;
__u8 name_len;
__u8 file_type;
char name[0];
char name[];
};

/*
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