| Message ID | 20200507185601.GA14759@embeddedor (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | modpost: Replace zero-length array with flexible-array | expand |
On Fri, May 8, 2020 at 3:51 AM Gustavo A. R. Silva <gustavoars@kernel.org> 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> Applied to linux-kbuild. Thanks.
diff --git a/scripts/basic/fixdep.c b/scripts/basic/fixdep.c index 877ca2c88246..d98540552941 100644 --- a/scripts/basic/fixdep.c +++ b/scripts/basic/fixdep.c @@ -160,7 +160,7 @@ struct item { struct item *next; unsigned int len; unsigned int hash; - char name[0]; + char name[]; }; #define HASHSZ 256 diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c index 5c3c50c5ec52..4d4b979d76be 100644 --- a/scripts/mod/modpost.c +++ b/scripts/mod/modpost.c @@ -166,7 +166,7 @@ struct symbol { * (only for external modules) **/ unsigned int is_static:1; /* 1 if symbol is not global */ enum export export; /* Type of export */ - char name[0]; + char name[]; }; static struct symbol *symbolhash[SYMBOL_HASH_SIZE]; diff --git a/scripts/mod/modpost.h b/scripts/mod/modpost.h index 60dca9b7106b..39f6c29fb568 100644 --- a/scripts/mod/modpost.h +++ b/scripts/mod/modpost.h @@ -111,7 +111,7 @@ buf_write(struct buffer *buf, const char *s, int len); struct namespace_list { struct namespace_list *next; - char namespace[0]; + char namespace[]; }; struct module {
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> --- scripts/basic/fixdep.c | 2 +- scripts/mod/modpost.c | 2 +- scripts/mod/modpost.h | 2 +- 3 files changed, 3 insertions(+), 3 deletions(-)