On 8/7/24 4:46 PM, Boqun Feng wrote:
On Wed, Aug 07, 2024 at 10:50:32AM +0200, Alice Ryhl wrote:
On Tue, Aug 6, 2024 at 9:30 PM Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
On Tue, Aug 06, 2024 at 10:48:11AM +0200, Alice Ryhl wrote:
[...]
+ /// Returns the flags associated with the file.
+ ///
+ /// The flags are a combination of the constants in [`flags`].
+ #[inline]
+ pub fn flags(&self) -> u32 {
+ // This `read_volatile` is intended to correspond to a READ_ONCE call.
+ //
+ // SAFETY: The file is valid because the shared reference guarantees a nonzero refcount.
+ //
+ // FIXME(read_once): Replace with `read_once` when available on the Rust side.
Do you know the status of this?
It's still unavailable.
I think with our own Atomic API, we can just use atomic_read() here:
yes, I know that to make this is not a UB, we need the C side to also do
atomic write on this `f_flags`, however, my reading of C code seems to
suggest that FS relies on writes to this field is atomic, therefore
unless someone is willing to convert all writes to `f_flags` in C into
a WRITE_ONCE(), nothing more we can do on Rust side. So using
atomic_read() is the correct thing to begin with.
Huh? The C side uses atomic reads for this?
Well, READ_ONCE(->f_flags) is atomic, so I thought you want to use
atomic here. However, after a quick look of `->f_flags` accesses, I find
out they should be protected by `->f_lock` (a few cases rely on
data race accesses, see p4_fd_open()), so I think what you should really
do here is the similar: make sure Rust code only accesses `->f_flags`
if `->f_lock` is held. Unless that's not the case for binder?
Binder just has an `if (filp->f_flags & O_NONBLOCK)` block somewhere in
the ioctl, where filp is the `struct file *` passed to the ioctl. Binder
doesn't take the lock.
Alice
