LWN: Comments on "Improving kernel string handling"

Improving kernel string handling

cesarb — Fri, 08 May 2015 18:49:28 +0000

For the kernel, -E2BIG is often what you want.

long sys_foo(...)
{
long ret = 0;

/* ... */

ret = strscpy(dest, src, sizeof_dest);
if (ret < 0)
goto err;
/* ret now has the string length, saving a strlen() */

/* ... */

return ret;

err:
/* ...cleanup... */
return ret;
}

That is, in case of error, the value can be returned directly to userspace. That is a common design pattern in the kernel: if a function you called returns a negative value (indicating failure), abort what you were doing and pass that value up the stack.

Improving kernel string handling

reubenhwk — Fri, 08 May 2015 18:08:45 +0000

I'd also suggest that -E2BIG is a really bad return value. Rather return -(space_needed).

int rc = strscpy(dest, src, sizeof(dest));
if (rc < 0) {
   dest = malloc(-rc);
   strscpy(dest, src, -rc);
}

...or something like that anyway.

Improving kernel string handling

ibukanov — Thu, 07 May 2015 10:10:32 +0000

I think it would be better for strscpy to require that count should be at least 1 treating count==0 as a programming error with similar consequences as passing a null pointer. This way one can be assured that the result is always null-terminated no matter what.