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Re: patch for mmap and friends
From: |
Maxime Devos |
Subject: |
Re: patch for mmap and friends |
Date: |
Sat, 14 Jan 2023 23:42:30 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.6.0 |
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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