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@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...
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 -- Problem reports: https://cygwin.com/problems.html FAQ: https://cygwin.com/faq/ Documentation: https://cygwin.com/docs.html Unsubscribe info: https://cygwin.com/ml/#unsubscribe-simple
