Re: [PATCH v2 3/3] posix: Implement preadv2 and pwritev2

[Zack Weinberg <zackw at panix dot com>]

On Tue, May 2, 2017 at 1:37 PM, Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
> On 02/05/2017 11:37, Christoph Hellwig wrote:
>> On Tue, May 02, 2017 at 11:33:32AM -0300, Adhemerval Zanella wrote:
>>> So default implementation just ignore the flag argument and calls generic
>>> preadv/pwritev.
>>
>> Please don't do that, it's actively dangerous.  Just fail the call.
>
> This a misleading commit comment from my part (I forgot to actually update
> the commit log).  In fact, I am following the suggestion from Zack and
> returning EINVAL for unsupported flags in generic implementation (which
> are currently none).

This is true for the stub implementation (always returns -1/ENOSYS)
and the sysdeps/posix implementation (returns -1/EINVAL if flags is
nonzero, else calls preadv/pwritev, which is the Right Thing) ... but
the sysdeps/unix/sysv/linux implementation appears to do the wrong
thing when kernel support isn't available:

+ssize_t
+preadv2 (int fd, const struct iovec *vector, int count, off_t offset,
+ int flags)
+{
+# ifdef __NR_preadv2
+  ssize_t result = SYSCALL_CANCEL (preadv2, fd, vector, count,
+   LO_HI_LONG (offset), flags);
+  if (result >= 0 || errno != ENOSYS)
+    return result;
+# endif
+  /* Trying to emulate the preadv2 syscall flags is troublesome:
+
+     * We can not temporary change the file state of the O_DSYNC and O_SYNC
+       flags to emulate RWF_{D}SYNC (attempts to change the state of using
+       fcntl silently ignored).
+
+     * IOCB_HIPRI requires the file opened in O_DIRECT and uses an internal
+       semantic not provided by any other flag (O_NONBLOCK for instance).
+
+     So for generic case just ignore the flags can call preadv directly.  */
+  return preadv (fd, vector, count, offset);
+}

The fallback logic needs to check whether flags is nonzero before
calling preadv.

(Relying on the kernel to reject unrecognized flags, when the syscall
*is* available, seems reasonable to me.)

zw