On 29/08/2019 14.15, Aleksa Sarai wrote:
On 2019-08-24, Daniel Colascione dancol@google.com wrote:
Why pad the structure when new functionality (perhaps accommodated via a larger structure) could be signaled by passing a new flag? Adding reserved fields to a structure with a size embedded in the ABI makes a lot of sense --- e.g., pthread_mutex_t can't grow. But this structure can grow, so the reservation seems needless to me.
Quite a few folks have said that ->reserved is either unnecessary or too big. I will be changing this, though I am not clear what the best way of extending the structure is. If anyone has a strong opinion on this (or an alternative to the ones listed below), please chime in. I don't have any really strong attachment to this aspect of the API.
There appear to be a few ways we can do it (that all have precedence with other syscalls):
- Use O_* flags to indicate extensions.
- A separate "version" field that is incremented when we change.
- Add a size_t argument to openat2(2).
- Reserve space (as in this patchset).
(My personal preference would be (3), followed closely by (2).)
3, definitely, and instead of having to invent a new scheme for every new syscall, make that the default pattern by providing a helper
int __copy_abi_struct(void *kernel, size_t ksize, const void __user *user, size_t usize) { size_t copy = min(ksize, usize);
if (copy_from_user(kernel, user, copy)) return -EFAULT;
if (usize > ksize) { /* maybe a separate "return user_is_zero(user + ksize, usize - ksize);" helper */ char c; user += ksize; usize -= ksize; while (usize--) { if (get_user(c, user++)) return -EFAULT; if (c) return -EINVAL; } } else if (ksize > usize) { memset(kernel + usize, 0, ksize - usize); } return 0; } #define copy_abi_struct(kernel, user, usize) \ __copy_abi_struct(kernel, sizeof(*kernel), user, usize)
Both (1) and (2) have the problem that the "struct version" is inside the struct so we'd need to copy_from_user() twice. This isn't the end of the world, it just feels a bit less clean than is ideal. (3) fixes that problem, at the cost of making the API slightly more cumbersome to use directly (though again glibc could wrap that away).
I don't see how 3 is cumbersome to use directly. Userspace code does struct openat_of_the_day args = {.field1 = x, .field3 = y} and passes &args, sizeof(args). What does glibc need to do beyond its usual munging of the userspace ABI registers to the syscall ABI registers?
Rasmus