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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