Re: [V9fs-developer] [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation

Theodore Y. Ts'o wrote on Fri, Dec 28, 2018:

On Sat, Dec 29, 2018 at 03:37:21AM +0100, Dominique Martinet wrote:

quoted

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Are there going to be cases where a process or a thread will sometimes
want the 64-bit interface, and sometimes want the 32-bit interface?
Or is it always going to be one or the other?  I wonder if we could
simply add a new flag to the process personality(2) flags.

That would likely work for qemu user, but the qemu system+9p case is
going to be more painful..
More precisely, the 9p protocol does not plan for anything other than
64bit offset so if the vfs needs to hand out a 32bit offset we'll need
to make a correspondance table between the 32bit offsets we hand off and
the 64bit ones to use; unless some flag can be passed at lopen to tell
the server to always hand out 32bit offsets for this directory... And if
we do that then 9p servers will need a way to use both APIs in parallel
for both types of directories.

How about if we add a fcntl(2) mediated flag, which is tied to a
struct file?  Would that be more or less painful for 9p and qemu
system+9p?

Hmm. 9P2000.L doesn't have anything akin to fcntl either, the only two
obvious places where we could pass a flag is lopen (which already
handles a bunch of linux-specific flags, e.g. passing O_LARGEFILE
O_NOATIME etc will just forward these through for qemu/diod at least),
or adding a new parameter to the 9p readdir.

The former would let us get away without modifying the protocol as
servers will just ignore flags they don't handle on implementations I
checked, so it'd definitely be the least effort choice from what I can
tell.


On the other hand a fcntl would solve the server-side problem, it'd
allow the server to request appropriately-sized offsets per fd, so it's
a good start; we "just" need to figure how to translate that on the wire.

-- 
Dominique Martinet | Asmadeus