Re: patch for mmap and friends

[Maxime Devos <maximedevos@telenet.be>]

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On 14-01-2023 23:08, Matt Wette wrote:
> 2) had to copy/modify dynwind_acquire_port and release_port from ports.c

Is it because of the 'static' qualifier?  If so, you could use the 
'SCM_INTERNAL [...] scm_i_[...]' pattern, e.g. see 
'scm_i_is_mutable_bitvector' in libguile/bitvectors.h and 
libguile/bitvectors.c.


>    if (SCM_UNBNDP (file))
>     c_fd = -1;
>   else {
>     /* Use the fd under clobber protection from GC or another thread. */
>     if (SCM_PORTP (file))
>       c_fd = scm_to_int (scm_fileno (file));
>     else {
>       c_fd = scm_to_int (file);
>       file = SCM_CAR (scm_fdes_to_ports (file));
>     }
>     scm_dynwind_acquire_port (file);
>   }

You need to acquire the port _before_ taking its file descriptor! 
Otherwise, it is protected too late.  I.e., scm_dynwind_acquire_port 
needs to be moved before the 'scm_fileno'.

On the second branch, in particular 'c_fd = scm_to_int (file);':
IIUC, the idea is to, when a raw fd is passed, look up the corresponding 
port to lock it, right?

If so, I think it's too late for that -- another thread might change 
things between 'c_fd = ...' and 'file = SCM_CAR (scm_fdes_to_ports (file))'.

More generally, when a raw fd is passed, I think it's impossible to 
solve the 'other thread/GC interfering' problem.

As such, to simplify things, I propose to only do the 
'scm_dynwind_acquire_port' when a port is passed, instead of failing to 
solve the interference problems (if the user passed a raw fd, then only 
they can make sure there are no problems, e.g. by changing their code to 
use ports or by not using move->fdes stuff).:

Proposed code (untested):

{
   void *c_mem, *c_addr;
   size_t c_len;
   int c_prot, c_flags, c_fd;
   scm_t_off c_offset;
   SCM pointer, bvec;

   if (SCM_POINTER_P (addr))
     c_addr = SCM_POINTER_VALUE (addr);
   else if (scm_is_integer (addr))
     c_addr = (void*) scm_to_uintptr_t (addr);
   else
     scm_misc_error ("mmap", "bad addr", addr);

   c_len = scm_to_size_t (len);

   if (SCM_UNBNDP (prot))
     c_prot = PROT_READ | PROT_WRITE;
   else
     c_prot = scm_to_int (prot);

   if (SCM_UNBNDP (flags))
     c_flags = MAP_ANONYMOUS | MAP_PRIVATE;
   else
     c_flags = scm_to_int (flags);

   scm_dynwind_begin (0);
   if (SCM_UNBNDP (file))
     c_fd = -1;
   else if (scm_is_integer (file))
     c_fd = scm_to_int (file);
   else
     {
       /* Use the fd of the port under clobber protection from
          concurrency. As scm_dynwind_acquire_port assumes that
          FILE is a port, check that first. */
       SCM_VALIDATE_PORT (SCM_ARG5, file);
       scm_dynwind_acquire_port (file);
       c_fd = scm_fileno (file);
     }

   if (SCM_UNBNDP (offset))
     c_offset = 0;
   else
     c_offset = scm_to_off_t (offset);

   if ((c_addr == NULL) && (c_flags & MAP_FIXED))
     scm_misc_error ("mmap", "cannot have NULL addr w/ MAP_FIXED", SCM_EOL);

   SCM_SYSCALL (c_mem = mmap(c_addr, c_len, c_prot, c_flags, c_fd, 
c_offset));
   if (c_mem == MAP_FAILED)
     scm_syserror ("mmap");              /* errno set */

   /* The fd is free to go now. */
   scm_dynwind_end ();

   pointer = scm_cell (scm_tc7_pointer, (scm_t_bits) c_mem);
   bvec = scm_c_take_typed_bytevector((signed char *) c_mem + c_offset, 
c_len,
                      SCM_ARRAY_ELEMENT_TYPE_VU8, pointer);
   scm_i_set_finalizer (SCM2PTR (bvec), mmap_finalizer, (void*) c_len);
   return bvec;
}
#undef FUNC_NAME



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