Will all SIDs fit into |SECURITY_MAX_SID_SIZE| bytes ? / was: Re: Switching groups with newgrp - how to get the new group with |GetTokenInformation()| ?

[Roland Mainz]

On Sat, Feb 24, 2024 at 7:57 PM Corinna Vinschen via Cygwin
<cygwin@cygwin.com> wrote:
>
> On Feb 24 15:38, Roland Mainz via Cygwin wrote:
> > On Thu, Feb 22, 2024 at 8:11 PM Corinna Vinschen via Cygwin
> > <cygwin@cygwin.com> wrote:
> > > On Feb 22 18:38, Roland Mainz via Cygwin wrote:
> > > > If I switch the current user's group with /usr/bin/newgrp, how can a
> > > > (native) Win32 process use
> > > > |GetTokenInformation(GetCurrentThreadToken(), ...)| to find out which
> > > > group is the new "current group" (e.g. which |TokenInformationClass|
> > > > should I use) ?
> > >
> > >   PSID sidbuf = (PSID) alloca (SECURITY_MAX_SID_SIZE);
> > [snip]
> >
> > Win32/NT API question: All known SIDs will fit into
> > |SECURITY_MAX_SID_SIZE| bytes, right ? I'm asking because right now
> > the ms-nfs41-client code assumes that all SIDs use a variable amount
> > of memory, and we always have to ask the Win32/NT API about the number
> > of bytes to allocate. If |SECURITY_MAX_SID_SIZE| is the global maximum
> > limit for all Windows versions, then we could simplify the code a
> > lot...
>
> Yes.  ACLs are size restricted to 64K, though, but that shouldn't be
> much of a problem usally.

Erm... why ACLs? I was asking about the memory allocation size for an SID.

Example:
Right now our code uses two calls to |LookupAccountNameA()| for the
conversion from a Windows account name to Windows SID.
The first call gets the allocation size for a SID, our code then
allocates that memory, and then does a second call to
|LookupAccountNameA()| to fill that memory with that SID.

If |SECURITY_MAX_SID_SIZE| (currently 68 bytes) is the maximum memory
size a Windows syscall can return for a SID, then the code above could
be simplified to a |sidmem =
malloc(SECURITY_MAX_SID_SIZE)|+|LookupAccountNameA(..., sidmem, ...)|.

The same could be done in many many other places, leading to a
considerable reduction of Win32 system library calls.

Question is whether the assumption about |SECURITY_MAX_SID_SIZE| is correct...

----

Bye,
Roland
-- 
  __ .  . __
 (o.\ \/ /.o) roland.mainz@nrubsig.org
  \__\/\/__/  MPEG specialist, C&&JAVA&&Sun&&Unix programmer
  /O /==\ O\  TEL +49 641 3992797
 (;O/ \/ \O;)