Re: [PATCH 032/104] virtiofsd: passthrough_ll: create new files in caller's context

[Daniel P . Berrangé]

On Mon, Jan 06, 2020 at 07:08:43PM +0000, Dr. David Alan Gilbert wrote:
> * Dr. David Alan Gilbert (address@hidden) wrote:
> > * Daniel P. Berrangé (address@hidden) wrote:
> > > On Thu, Dec 12, 2019 at 04:37:52PM +0000, Dr. David Alan Gilbert (git) 
> > > wrote:
> > > > From: Vivek Goyal <address@hidden>
> > > > 
> > > > We need to create files in the caller's context. Otherwise after
> > > > creating a file, the caller might not be able to do file operations on
> > > > that file.
> > > > 
> > > > Changed effective uid/gid to caller's uid/gid, create file and then
> > > > switch back to uid/gid 0.
> > > > 
> > > > Use syscall(setresuid, ...) otherwise glibc does some magic to change 
> > > > EUID
> > > > in all threads, which is not what we want.
> > > > 
> > > > Signed-off-by: Vivek Goyal <address@hidden>
> > > > Signed-off-by: Miklos Szeredi <address@hidden>
> > > > ---
> > > >  tools/virtiofsd/passthrough_ll.c | 79 ++++++++++++++++++++++++++++++--
> > > >  1 file changed, 74 insertions(+), 5 deletions(-)
> > > > 
> > > > diff --git a/tools/virtiofsd/passthrough_ll.c 
> > > > b/tools/virtiofsd/passthrough_ll.c
> > > > index 68bacb6fc5..0188cd9ad6 100644
> > > > --- a/tools/virtiofsd/passthrough_ll.c
> > > > +++ b/tools/virtiofsd/passthrough_ll.c
> > > 
> > > 
> > > > +static int lo_change_cred(fuse_req_t req, struct lo_cred *old)
> > > > +{
> > > > +    int res;
> > > > +
> > > > +    old->euid = geteuid();
> > > > +    old->egid = getegid();
> > > > +
> > > > +    res = syscall(SYS_setresgid, -1, fuse_req_ctx(req)->gid, -1);
> > > 
> > > Do we need to be using  SYS_setres[u,g]id32 instead...
> > > 
> > > [quote setresgid(2)]
> > >        The original Linux setresuid() and setresgid() system  calls
> > >        supported  only  16-bit  user  and group IDs.  Subsequently,
> > >        Linux 2.4 added setresuid32() and setresgid32(),  supporting
> > >        32-bit  IDs.   The glibc setresuid() and setresgid() wrapper
> > >        functions transparently deal with the variations across ker‐
> > >        nel versions.
> > > [/quote]
> > 
> > OK, updated.
> 
> Hmm hang on; this is messy.  x86-64 only seems to have setresuid
> where as some architectures have both;  If I'm reading this right, all
> 64 bit machines have setresuid/gid calling the code that takes the
> 32bit ID; some have compat entries for 32bit syscalls.

Oh yuk.

> I think it's probably more correct to call setresuid here; except
> for 32 bit platforms - but how do we tell?

Is it possible to just do an #ifdef SYS_setresgid32 check to see
if the wider variant exists ?


Regards,
Daniel
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