[Bug 1785902] Re: local/9pfs: Too many levels of symbolic links

[Christian Schoenebeck]

Enrico, with security_model=mapped (a.k.a. security_model=mapped-xattr)
9p is not following symlinks on host. That's the expected behaviour.

There are 2 distinct models:

security_model=passthrough uses the ownership information, permissions
and symlink info etc. directly from the host's file system. This model
requires the qemu binary to run as high privileged user (i.e. root)
though, because it needs to be able to change file ownership,
permissions and so forth. For that reason this model is only appropriate
for use cases where there is a certain trust to what the guest system is
doing. E.g. this model is commonly used by kernel coders to build
kernel/driver code on host and then test run it as guest.

For untrusted guests (i.e. most cases) it is recommended to use
security_model=mapped-xattr instead. In this mode all ownership
information, permissions, etc. are emulated (i.e. mapped) by storing
them as extended attributes on top of files on host. This way the qemu
binary can run with a regular (unprivileged) user and pretend to host it
would have all rights to change ownership and permissions.

You can read more details about this in the 9p developer description about the 
9p 'local' driver here:
https://wiki.qemu.org/Documentation/9p#9p_Filesystem_Drivers

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https://bugs.launchpad.net/bugs/1785902

Title:
  local/9pfs: Too many levels of symbolic links

Status in QEMU:
  Incomplete

Bug description:
  Version: 2.9.1

  The primary symptom is resolving symlink fails w/ error "too many
  levels of symbolic links".

  My analysis showed that local_readlink() uses local_open_nofollow() to
  open the file and then tries to read it. local_open_nofollow() then
  tries to open the file w/ O_NOFOLLOW, which obviously fails if the
  requested file is a symlink.

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