Re: [patch 7/8] fdmap v2 - implement sys_socket2

[Linus Torvalds]

On Wed, 6 Jun 2007, Alan Cox wrote:
> 
> This still all seems really really ugly.

I do agree that it's ugly. That many new system calls with new prototypes 
and new glibc support is just nasty.

So I don't think this is viable.

> Is there anything wrong with throwing all these extra cases out and 
> replacing the entire lot with
> 
> 	prctl(PR_SPARSEFD, 1);
> 
> to turn on sparse fd allocation for a process ?

Yes. We really don't want to set global state that affects any random 
library thing that runs after it.

HOWEVER.

I think we could introduce a *single* new system call, which does 
basically a "run the specified system call with the following flags".

The flags would literally be local to that *one* system call, and one of 
the flags could be the semantics for FD allocation.

[ There are a few other cases where such an indirect system call might be 
  interesting: temporarily unmasking a signal for just the duration of a 
  single system call is the reason for things like 'pselect()' and 
  'sigtimedwait()', and similarly the 'access()' system call is basically 
  a "temporarily run with my real UID, rather than the effective UID 
  thing, and quite frankly, it might be perfectly valid to want to do an 
  'open()' with that rule too, because "access()+open()" is racy! ]

So maybe the proper solution to this mess is *not* to add fifteen new 
system calls, but to add *one*, which takes a "flags" value to set certain 
things:

 - FD_NONSEQ: "allocate any new fd's nonsequentially"
 - FD_CLOEXEC: "allocate any new fd's as close-on-exec"

   Rationale: allow people to open any fd with the flags set a certain 
   way, regardless of the system call.

 - LOOKUP_REALUID/GID: "make the fsuid/fsgid temporarily be my _real_ 
   uid/gid for this single system call"

   Rationale: avoid the inevitable races that the fundamentally broken 
   "access()" system call has! 

 - LOOKUP_NOFOLLOW: "do not follow any symlink at the end of the path"
   LOOKUP_NOATIME: "don't update atime"

   Rationale: "open()" already has O_NOFOLLOW/O_NOATIME, and "stat()" has 
   "lstat()", but a lot of other path-handlign system calls cannot do the 
   same thing.

 - LOOKUP_NOSYMLINKS: "do not allow any symlink traversal at *all*"
   LOOKUP_NODOTDOT: "don't traverse a .. upwards"
   LOOKUP_NOMOUNT: "don't traverse a mount point"

   Rationale: for security-conscious things, quite often it's not the 
   _last_ symlink you want to avoid, it's any symlinks at all, and 
   sometimes it's things like guaranteeing that you stay in a certain 
   directory structure - which means not going outside with ".." or some 
   magic mount-point.

   People currently literally end up traversing things one path component 
   at a time, doing a "lstat()" on it, and checking. Even if 99% 
   of the time you probably don't actually ever hit the problem case. 
   (Eg Apache at some point used to do something like this if you asked 
   for security, I'm not sure if it still does).

 - signal mask for temporarily blocking/unblocking during a single system 
   call.

 - something else? The above are things that I know I _personally_ have 
   occasionally cursed not having had.

What do people think about that kind of approach? It has the advantage 
that it does *not* involve multiple kernel entries (just a single entry to 
a small wrapper that sets some process state temporarily), and that it 
doesn't have any sticky state that might confuse a library (or a signal 
handler: even if you end up doing "prctrl(ON) ; syscall(); prctrl(OFF)", a 
signal handler that happens in between the prctrl's would see unexpected 
behaviour).

It has the disadvantage that it would need some per-architecture setup to 
load the actual real arguments from memory: the system call would probably 
look something like

	syscall_indirect(unsigned long flags, sigset_t *, 
			 int syscall, unsigned long args[6]);

and the rule would be that it would just load the six system call 
registers from that "args[]" array. Always load the full six registers, to 
make it simpler and faster, and not having any confusion or ever needing 
any wrappers that depend on the number of system calls.

			Linus
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