struct_size() using sizeof() vs offsetof()

[Alejandro Colomar]

Hi Kees, Gustavo,

I've been discussing with a friend about the appropriateness of sizeof()
vs offsetof() for calculating the size of a structure with a flexible
array member (FAM).

After reading Jens Gustedt's blog post about it[1], we tried some tests,
and we got some interesting results that discouraged me from using sizeof().
See below.

But then, said friend pointed to me that the kernel uses sizeof() in
struct_size(), and we wondered why you would have chosen it.  It's safe
as long as you _know_ that there's no padding, or that the alignment of
the FAM is as large as the padding (which you probably know in the kernel),
but it seems safer to use

	MAX(sizeof(s), offsetof(s, fam) + sizeof_member(s, fam) * count)

The thing is, if there's any trailing padding in the struct, the FAM may
overlap the padding, and the calculation with sizeof() will waste a few
bytes, and if misused to get the location of the FAM, the problem will be
bigger, as you'll get a wrong location.

So, I just wanted to pry what and especially why the kernel chose to prefer
a simple sizeof().

Cheers,
Alex

---

$ cat off.c 
#include <err.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>


struct s {
	int   i;
	char  c;
	char  fam[];
};


static inline void *xmalloc(size_t size);


int
main(void)
{
	char      *p;
	struct s  *s;

	printf("sizeof: %zu\n", sizeof(struct s));
	printf("offsetof: %zu\n", offsetof(struct s, fam));

	puts("\nWith sizeof():");

	s = xmalloc(sizeof(struct s) + sizeof("Hello, sizeof!"));
	strcpy(s->fam, "Hello, sizeof!");
	p = (char *) s + sizeof(struct s);
	puts(p);
	free(s);

	puts("\nWith offsetof(3):");

	s = xmalloc(offsetof(struct s, fam) + sizeof("Hello, offsetof!"));
	strcpy(s->fam, "Hello, offsetof!");
	p = (char *) s + offsetof(struct s, fam);
	puts(p);
	free(s);

	exit(EXIT_SUCCESS);
}


static inline void *
xmalloc(size_t size)
{
	void  *p;

	p = malloc(size);
	if (p == NULL)
		err(EXIT_FAILURE, "malloc");
	return p;
}


$ ./a.out 
sizeof: 8
offsetof: 5

With sizeof():
lo, sizeof!

With offsetof(3):
Hello, offsetof!


[1]: <https://gustedt.wordpress.com/2011/03/14/flexible-array-member/>
-- 
<http://www.alejandro-colomar.es/>
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