Re: Using thread-specific credentials on Linux in Samba with glibc.

[Jeremy Allison <jra at samba dot org>]

On Mon, Jul 02, 2012 at 03:27:23PM -0700, Roland McGrath wrote:
> > Sure, no problem. I want to be able to use this function:
> > 
> > ----------------------------------------------------------
> > int samba_setresuid(uid_t ruid, uid_t euid, uid_t suid)
> > {
> >         return syscall(SYS_setresuid, ruid, euid, suid);
> > }
> > ----------------------------------------------------------
> 
> That's certainly always going to be fine unless there is a change to the
> calling convention of the syscall in question.  All libc says about
> 'syscall' is that it makes the system call with the number you specified
> and fills the argument registers with the arguments you gave.  The rest
> of what that means is between you and the kernel.  For example, if the
> syscall were to change to take 64-bit arguments on 32-bit platforms,
> then the argument encoding would not be reliably correct on all
> machines.  But that's existing libc behavior on the affected platforms:
> it expects word-sized arguments and passes them along in the
> corresponding registers.  Some machines have argument-passing
> conventions for larger-sized arguments that make this not do what you
> really wanted it to do, e.g. a single argument twice the word size going
> into an even/odd register pair, but the 'syscall' function takes an
> additional leading argument (the number) that the system call itself
> doesn't take, fouling up the even/odd sense of particular argument registers.

Ok, thanks. Much appreciated. If there's a change to the kernel
to break the ABI we have much bigger problems that this anyway :-).

Cheers and thanks for the clarification,

Jeremy.