avoid sending pointer values in struct passwd

[Thorsten Glaser]

On Wed, 25 Nov 2020, Damien Miller wrote:

> +		    (r = sshbuf_put_u64(b, pwent->id < 0 ? \
> +		    -pwent->id : pwent->id)) != 0) \

*cough* -2⁶³ will trigger UB/IB on this.


Since sender and recipient are on the same system, you can just…

union a64bitquantity {
	uint64_t u;
	int64_t i;
}

#define PUTPW(b, id) do {				\
	unsigned char issigned = pwent->id < 0;		\
	union a64bitquantity q;				\
							\
	if (issigned)					\
		q.i = pwent->id;			\
	else						\
		q.u = pwent->id;			\
							\
	if ((r = sshbuf_put_u8(b, issigned)) != 0 ||	\
	    (r = sshbuf_put_u64(b, q.u)) != 0)		\
		fatal_fr(r, "assemble pw %s", #id);	\
} while (/* CONSTCOND */ 0)

#define GETPW(b, id) do {				\
	unsigned char issigned;				\
	union a64bitquantity q;				\
							\
	if ((r = sshbuf_get_u8(b, &issigned)) != 0 ||	\
	    (r = sshbuf_get_u64(b, &q.u)) != 0)		\
		fatal_fr(r, "parse pw %s", #id);	\
	/* don’t use a ternary expression here */	\
	if (issigned)					\
		pwent->id = q.i;			\
	else						\
		pwent->id = q.u;			\
} while (/* CONSTCOND */ 0)

Use of such a union is AFAIR safe.

Using a ternary operation could trigger UB/IB by
trying to bring the operands to the same type.


It might actually be easier to try and find out whether
the type is signed or not. I don’t know of a good method,
“the ’net” suggests:

#define	ISUNSIGNED(T)	((T)-1 > 0)

This, unfortunately, doesn’t work as a compile-time check.
But we can do this during configure time:

~ $ gcc -DI='<stdlib.h>' -DT=size_t -c a.c
~ $ gcc -DI='<stdlib.h>' -DT=ssize_t -c a.c
a.c: In function ‘foo’:
a.c:4:6: error: size of array ‘statictest’ is negative
    4 |  int statictest[((T)-1 > 0) ? 1 : -1];
      |      ^~~~~~~~~~
1|~ $ cat a.c
#include I

int foo(void) {
	int statictest[((T)-1 > 0) ? 1 : -1];
	return (0);
}
~ $

Check whether this compiles to set preprocessor
definitions, then use the to select the suitable
function to pass the value.

This is what I feel is safest; ofc YMMV…

bye,
//mirabilos
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