Re: [Qemu-devel] [PATCH 1/6] qemu-bridge-helper: Fix misuse of isspace()

[Markus Armbruster <armbru@redhat.com>]

Peter Maydell <peter.maydell@linaro.org> writes:

> On Mon, 13 May 2019 at 14:21, Markus Armbruster <armbru@redhat.com> wrote:
>> Perhaps I should do it just for this file while I touch it anyway.  The
>> question to ask: should parse_acl_file() obey the locale for whitespace
>> recognition?
>
> I vote for "no".
>
> Q: do we document the format of the ACL file anywhere ?

Support for it was added in commit bdef79a2994, v1.1.  Just code, no
documentation.

Grepping for qemu-bridge-helper finds just qemu-options.hx.  Contains
-help output and some .texi that goes into qemu-doc and the manual page.
None of it mentions how qemu-bridge-helper is run, or the ACL file
feature, let alone what its format might be.

I'm afraid all we have is the commit message.  Which doesn't really
define the file format, it merely gives a bunch of examples.

As far as I can tell, qemu-bridge-helper is for use with -netdev tap and
-netdev bridge.

Both variations of -netdev call net_bridge_run_helper() to run the
helper.  First argument is -netdev parameter "helper", default usually
"$prefix/libexec/qemu-bridge-helper".  Second argument is parameter
"br", default "br0".

If @helper contains space or tab, net_bridge_run_helper() guesses its a
full command, else it guesses its the name of the executable.  Bad
magic.

If it guesses name of executable, it execv()s this executable with
arguments "--use-vnet", "--fd=FD", "--br=@bridge".

If it guesses full command, it appends "--use-vnet --fd=FD", where FD is
the helper's half of the socketpair used to connect QEMU and the helper.
It further appends "--br=@bridge", unless @helper contains "--br=".
More bad magic.

It executes the resulting string with sh -c.  Magic cherry on top.

When the helper fails, netdev creation fails.

The helper we ship with QEMU unconditionally tries to read
"$prefix/etc/bridge.conf".  Fatal error if this file doesn't exist.
Errors in this file are fatal.  Errors in files it includes are not
fatal; instead, the remainder of the erroneous file is ignored.
*Boggle*

As far as I can tell, libvirt runs qemu-bridge-helper itself (Paolo's
commit 2d80fbb14df).  Makes sense, because running QEMU with the
necessary privileges would be unwise, and so would be letting it execute
setuid helpers.  Also bypasses the bad magic in QEMU's
net_bridge_run_helper().

qemu-bridge-helper should have a manual page, and its handling of errors
in ACL include files needs work.  There's probably more; I just glanced
at it.  I'm not volunteering, though.  It lacks a maintainer.  Should we
add it to Jason's "Network device backends"?

-netdev's helper parameter is seriously underdocumented.  Document or
deprecate?