[0/4] 9pfs: Fix ftruncate-after-unlink

Message

Greg Kurz March 10, 2025, 5:10 p.m. UTC

QEMU 9.2 already fixed the long standing limitation of failing fstat() on
unlinked files. This series does something similar for ftruncate().

The following program can be straced inside the guest with a shared fs in
passthrough mode over 9p2000.L.

int main(void)
{
	struct stat st;
	int fd = creat("./foo", 0000);

	ftruncate(fd, 100);
	unlink("./foo");
	ftruncate(fd, 1000);
}

Before :

creat("./foo", 000)                     = 3
ftruncate(3, 100)                       = -1 EACCES (Permission denied)
unlink("./foo")                         = 0
ftruncate(3, 1000)                      = -1 ENOENT (No such file or directory)

After :

creat("./foo", 000)                     = 3
ftruncate(3, 100)                       = 0
unlink("./foo")                         = 0
ftruncate(3, 1000)                      = 0

Christian,

I'm not familiar enough with the latest changes to write a proper test
for this case and I don't have enough cycles to learn. I'm sorry for that
but I guess it will be a lot easier for you and I'll review.

Cheers,

--
Greg

Greg Kurz (4):
  9pfs: local : Introduce local_fid_fd() helper
  9pfs: Don't use file descriptors in core code
  9pfs: Introduce ftruncate file op
  9pfs: Introduce futimens file op

 fsdev/file-op-9p.h |  5 +++++
 hw/9pfs/9p-local.c | 49 ++++++++++++++++++++++++++++++++++------------
 hw/9pfs/9p-synth.c | 22 +++++++++++++++++++++
 hw/9pfs/9p-util.h  |  1 +
 hw/9pfs/9p.c       | 21 +++++++++++++++-----
 hw/9pfs/cofs.c     | 37 ++++++++++++++++++++++++++++++++++
 hw/9pfs/coth.h     |  4 ++++
 7 files changed, 122 insertions(+), 17 deletions(-)