Re: [wish] Flexible array members in unions

[Alejandro Colomar <alx.manpages@gmail.com>]

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[CC += Kees, Andrew]

[start of thread: <https://inbox.sourceware.org/gcc/ac2550e0-2f5c-3680-08e6-0c224d043036@gmail.com/T/#u>]

On 5/11/23 18:07, Alejandro Colomar wrote:
> Hi!
> 
> Currently, one can have pseudo-flexible array members in unions with
> [0] syntax, but it's not allowed with [] syntax.
> 
> Here's an example of how it is possible today:
> 
> struct s {
> 	...
> 
> 	size_t  n;
> 	union {
> 		ptrdiff_t  off[0];  // [n]; offsets from s->data.
> 		char       data[0];
> 	};
> };
> 
> which is useful to have a structure with two (or really several)
> consecutive flexible arrays: one of offsets, which mark the positions
> of data, and another with the actual data.  Below goes an example
> program, which works fine with GCC, and I believe rewriting it to
> not use the union would make it less clear, since I'd need to add
> casts to it.
> 
> It works thanks to [0] pseudo-flexible arrays, but it doesn't
> compile with C99 flexible arrays.  And of course, [0] arrays have
> issues with -fstrict-flex-arrays=3.
> 
> 
> $ cat flexi4.c 
> #include <stddef.h>
> #include <stdlib.h>
> #include <stdio.h>
> #include <string.h>
> 
> struct s {
> 	size_t     n;
> 	union {
> 		ptrdiff_t  off[0];
> 		char       data[0];
> 	};
> };
> 
> int
> main(void)
> {
> 	char      *p;
> 	struct s  *s;
> 
> 	s = malloc(offsetof(struct s, off) +
> 	           sizeof(ptrdiff_t) * 2 +
> 	           sizeof("foobar") + sizeof("baz"));
> 
> 	s->n = 2;
> 	p = s->data + sizeof(ptrdiff_t) * s->n;
> 
> 	s->off[0] = p - s->data;
> 	p = stpcpy(p, "foobar") + 1;
> 	s->off[1] = p - s->data;
> 	p = stpcpy(p, "baz") + 1;
> 
> 	puts(s->data + s->off[0]);
> 	puts(s->data + s->off[1]);
> 
> 	free(s);
> }
> $ gcc-13 -Wall -Wextra -Werror -fanalyzer \
>          -fsanitize=undefined -fsanitize=address \
>          -D_FORTIFY_SOURCE=3 -fstrict-flex-arrays=2 \
>          flexi4.c 
> $ ./a.out 
> foobar
> baz
> $ gcc-13 -Wall -Wextra -Werror -fanalyzer \
>          -fsanitize=undefined -fsanitize=address \
>          -D_FORTIFY_SOURCE=3 -fstrict-flex-arrays=3 \
>          flexi4.c 
> $ ./a.out 
> flexi4.c:27:8: runtime error: index 0 out of bounds for type 'ptrdiff_t [*]'
> flexi4.c:29:8: runtime error: index 1 out of bounds for type 'ptrdiff_t [*]'
> flexi4.c:32:23: runtime error: index 0 out of bounds for type 'ptrdiff_t [*]'
> foobar
> flexi4.c:33:23: runtime error: index 1 out of bounds for type 'ptrdiff_t [*]'
> baz
> 
> 
> Would you allow flexible array members in unions?  Is there any
> strong reason to disallow them?
> 
> Currently, I get:
> 
> $ gcc-13 -Wall -Wextra -fanalyzer \
>          -fsanitize=undefined -fsanitize=address \
>          -D_FORTIFY_SOURCE=3 -fstrict-flex-arrays=3 \
>          flexi4-true.c 
> flexi4-true.c:9:28: error: flexible array member in union
>     9 |                 ptrdiff_t  off[];
>       |                            ^~~
> flexi4-true.c:10:28: error: flexible array member in union
>    10 |                 char       data[];
>       |                            ^~~~
> 
> 

Currently, the Linux kernel has to go through some hoops due to this
restriction:


$ grepc -tm __DECLARE_FLEX_ARRAY *
include/uapi/linux/stddef.h:42:
#define __DECLARE_FLEX_ARRAY(TYPE, NAME)	\
	struct { \
		struct { } __empty_ ## NAME; \
		TYPE NAME[]; \
	}


tools/include/uapi/linux/stddef.h:42:
#define __DECLARE_FLEX_ARRAY(TYPE, NAME)	\
	struct { \
		struct { } __empty_ ## NAME; \
		TYPE NAME[]; \
	}
$ grep -rnB2 __DECLARE_FLEX_ARRAY * | head
include/uapi/rdma/rdma_user_rxe.h-153-	__u32			reserved;
include/uapi/rdma/rdma_user_rxe.h-154-	union {
include/uapi/rdma/rdma_user_rxe.h:155:		__DECLARE_FLEX_ARRAY(__u8, inline_data);
include/uapi/rdma/rdma_user_rxe.h:156:		__DECLARE_FLEX_ARRAY(__u8, atomic_wr);
include/uapi/rdma/rdma_user_rxe.h:157:		__DECLARE_FLEX_ARRAY(struct rxe_sge, sge);
--
include/uapi/scsi/scsi_netlink_fc.h-53-	union {
include/uapi/scsi/scsi_netlink_fc.h-54-		__u32 event_data;
include/uapi/scsi/scsi_netlink_fc.h:55:		__DECLARE_FLEX_ARRAY(__u8, event_data_flex);
--




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