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Date:	Sun, 25 Feb 2024 23:17:27 -0500
To:	Roland Mainz <roland DOT mainz AT nrubsig DOT org>
Cc:	cygwin AT cygwin DOT com
Subject:	Re: Will all SIDs fit into |SECURITY_MAX_SID_SIZE| bytes ? / was:
	Re: Switching groups with newgrp - how to get the new group with
	|GetTokenInformation()| ?
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On Sun, Feb 25, 2024 at 05:32:32PM -0500, Glenn Strauss via Cygwin wrote:
> On Sun, Feb 25, 2024 at 10:04:29PM +0100, Roland Mainz via Cygwin wrote:
> > On Sat, Feb 24, 2024 at 7:57 PM Corinna Vinschen via Cygwin
> > <cygwin AT cygwin DOT 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 AT cygwin DOT 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...
> 
> A robust solution which also reduces syscalls does not necessarily
> require a precise answer here.
> 
> I suggest writing a wrapper function which has on the stack
>   CSTR sidbuf[SECURITY_MAX_SID_SIZE];
> and calls LookupAccountNameA() passing sidbuf as Sid.
> If it succeeds, then malloc() returned cbSid value and copy sidbuf[].
> If it fails because the buffer is too small, then malloc() the returned
> cbSid value and call LookupAccountNameA() again.
> 
> Doing the above will keep memory use to a minimum, and will generally
> call LookupAccountNameA() once per wrapper func invocation rather than
> twice.
> 
> Cheers, Glenn

Commenters on stackoverflow provide data calculating the answer as
68 bytes for SID as binary [1]

[1] https://stackoverflow.com/questions/1140528/what-is-the-maximum-length-of-a-sid-in-sddl-format

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