Re: procfs files vs symlink

[RVP <rvp%SDF.ORG@localhost>]

On Wed, 12 Jan 2022, Robert Elz wrote:

>  | > What causes that EINVAL?
>  |
>  |
>  | I'm not sure (somneone suggested that the file descriptor has been closed
>  | when ls tries to fstat() it, but I can't confirm this).
>
> That should generate EBADF not EINVAL.  Attempting readlink()
> on something that is not a symlink, and various other possibilities
> like that would be more probable.  EINVAL isn't listed as a possible
> error return from [f]stat ... not that that guarantees that it cannot
> happen, particularly from within emulation code.

That someone is me. But, your're right about this. The EINVAL is caused by
using readlink() on what was a symlink, but, is not anymore because the fd
now points to a regular file.

ktrace ls -l /proc/self/fd

  1954   1954 ls       CALL  open(0x7d9270fdc000,0x600004,3)
  1954   1954 ls       NAMI  "/proc/self/fd"
  1954   1954 ls       RET   open 4

  1954   1954 ls       CALL  fchdir(4)

  1954   1954 ls       CALL  __getdents30(4,0x7d9270fa6000,0x1000)

  1954   1954 ls       CALL  __lstat50(0x7d9270fd12f0,0x7d9270fd12f8)
  1954   1954 ls       NAMI  "0"
  1954   1954 ls       RET   __lstat50 0
  1954   1954 ls       CALL  __lstat50(0x7d9270fd1430,0x7d9270fd1438)
  1954   1954 ls       NAMI  "1"
  1954   1954 ls       RET   __lstat50 0
  1954   1954 ls       CALL  __lstat50(0x7d9270fd1570,0x7d9270fd1578)
  1954   1954 ls       NAMI  "2"
  1954   1954 ls       RET   __lstat50 0
  1954   1954 ls       CALL  __lstat50(0x7d9270fd16b0,0x7d9270fd16b8)
  1954   1954 ls       NAMI  "3"
  1954   1954 ls       RET   __lstat50 0
  1954   1954 ls       CALL  __lstat50(0x7d9270fd17f0,0x7d9270fd17f8)
  1954   1954 ls       NAMI  "4"

Up to this point fd 4 is a symlink which is, presumably, being
cached by ls(1).

  1954   1954 ls       CALL  close(4)

passwd db now opened at 4 which now is a regular file.

  1954   1954 ls       CALL  open(0x7d92705b0006,0x400000,0)
  1954   1954 ls       NAMI  "/etc/pwd.db"
  1954   1954 ls       RET   open 4

Now ls(1) does the listing:

  1954   1954 ls       GIO   fd 1 wrote 44 bytes
       "crw--w----  1 rvp  tty  5, 2 Jan 13 04:05 0\n"
  1954   1954 ls       RET   write 44/0x2c
  1954   1954 ls       CALL  write(1,0x7d9270fa2000,0x2c)
  1954   1954 ls       GIO   fd 1 wrote 44 bytes
       "crw--w----  1 rvp  tty  5, 2 Jan 13 04:05 1\n"
  1954   1954 ls       RET   write 44/0x2c
  1954   1954 ls       CALL  write(1,0x7d9270fa2000,0x2c)
  1954   1954 ls       GIO   fd 1 wrote 44 bytes
       "crw--w----  1 rvp  tty  5, 2 Jan 13 04:05 2\n"

  1954   1954 ls       CALL  readlink(0x7f7fff425a30,0x7f7fff425e40,0x400)
  1954   1954 ls       NAMI  "/proc/self/fd/3"
  1954   1954 ls       RET   readlink 4
  1954   1954 ls       CALL  write(1,0x7d9270fa2000,0x34)
  1954   1954 ls       GIO   fd 1 wrote 52 bytes
       "lr-xr-xr-x  1 rvp  rvp   240 Jan 13 04:05 3 -> /tmp\n"

The readlink on 4 will fail because it is no longer the symlink it
originally was:

  1954   1954 ls       CALL  readlink(0x7f7fff425a30,0x7f7fff425e40,0x400)
  1954   1954 ls       NAMI  "/proc/self/fd/4"
  1954   1954 ls       RET   readlink -1 errno 22 Invalid argument


Anyway, here's one more patch:

---START PATCH---
--- sys/miscfs/procfs/procfs_vnops.c.orig	2022-01-12 20:57:25.944841288 +0000
+++ sys/miscfs/procfs/procfs_vnops.c	2022-01-12 23:07:44.736070587 +0000
@@ -1410,7 +1410,7 @@
 			d.d_fileno = PROCFS_FILENO(pfs->pfs_pid, PFSfd, i - 2);
 			d.d_namlen = snprintf(d.d_name, sizeof(d.d_name),
 			    "%lld", (long long)(i - 2));
-			d.d_type = VREG;
+			d.d_type = DT_UNKNOWN;	/* convert vtype -> DT_XXX ? */
 			if ((error = uiomove(&d, UIO_MX, uio)) != 0)
 				break;
 			if (cookies)
---END PATCH---

DT_UNKNOWN is not quite right either, but that's better than VREG
which corresponds to DT_FIFO.

-RVP