Applied, thanks!
Sergey Bugaev via Libc-alpha, le lun. 26 juin 2023 02:17:51 +0300, a ecrit:
> MAP_FIXED is defined to silently replace any existing mappings at the
> address range being mapped over. This, however, is a dangerous, and only
> rarely desired behavior.
>
> Various Unix systems provide replacements or additions to MAP_FIXED:
>
> * SerenityOS and Linux provide MAP_FIXED_NOREPLACE. If the address space
> already contains a mapping in the requested range, Linux returns
> EEXIST. SerenityOS returns ENOMEM, however that is a bug, as the
> MAP_FIXED_NOREPLACE implementation is intended to be compatible with
> Linux.
>
> * FreeBSD provides the MAP_EXCL flag that has to be used in combination
> with MAP_FIXED. It returns EINVAL if the requested range already
> contains existing mappings. This is directly analogous to the O_EXCL
> flag in the open () call.
>
> * DragonFly BSD, NetBSD, and OpenBSD provide MAP_TRYFIXED, but with
> different semantics. DragonFly BSD returns ENOMEM if the requested
> range already contains existing mappings. NetBSD does not return an
> error, but instead creates the mapping at a different address if the
> requested range contains mappings. OpenBSD behaves the same, but also
> notes that this is the default behavior even without MAP_TRYFIXED
> (which is the case on the Hurd too).
>
> Since the Hurd leans closer to the BSD side, add MAP_EXCL as the primary
> API to request the behavior of not replacing existing mappings. Declare
> MAP_FIXED_NOREPLACE and MAP_TRYFIXED as aliases of (MAP_FIXED|MAP_EXCL),
> so any existing software that checks for either of those macros will
> pick them up automatically. For compatibility with Linux, return EEXIST
> if a mapping already exists.
>
> Finally, a fun bit of horrifying trivia: until very recently, there has
> been a function named try_mmap_fixed () in the Wine project. On Darwin,
> it used mach_vm_map () to map the (anonymous) pages without replacing
> any existing mappings. On Solaris and NetBSD, it instead paused the
> other threads in the process using vfork (), then used mincore () to
> probe the pages in the desired address range for being already mapped --
> an error return from mincore () indicates that the page is not mapped at
> all. Finally, if no conflicting mappings were found, still in the
> vforked child process it performed the mapping with MAP_FIXED, and then
> returned from the child back into the parent, resuming the other
> threads.
>
> This relied on:
>
> * being able to do calls other than execve and _exit from the vforked
> child, which is undefined behavior according to POSIX;
>
> * the parent and the child sharing the address space, and changes made
> in the child being visible in the parent;
>
> * vfork suspending all the threads of the calling process, not just the
> calling thread.
>
> All of this is undefined according to POSIX, but was apparently true on
> Solaris, which is the system this function was originally implemented
> for. But on NetBSD, where this shim was later ported to, the last bullet
> point does not hold: vfork only suspends the calling thread; while the
> other threads continue to run.
>
> Signed-off-by: Sergey Bugaev <buga...@gmail.com>
> ---
> sysdeps/mach/hurd/bits/mman_ext.h | 7 ++++++-
> sysdeps/mach/hurd/mmap.c | 26 +++++++++++++++++---------
> 2 files changed, 23 insertions(+), 10 deletions(-)
>
> diff --git a/sysdeps/mach/hurd/bits/mman_ext.h
> b/sysdeps/mach/hurd/bits/mman_ext.h
> index bbb94743..9658cdd6 100644
> --- a/sysdeps/mach/hurd/bits/mman_ext.h
> +++ b/sysdeps/mach/hurd/bits/mman_ext.h
> @@ -22,5 +22,10 @@
>
> #ifdef __USE_GNU
> # define SHM_ANON ((const char *) 1)
> -# define MAP_32BIT 0x1000
> +
> +# define MAP_32BIT 0x1000 /* Map in the lower 2 GB. */
> +# define MAP_EXCL 0x4000 /* With MAP_FIXED, don't replace existing
> mappings. */
> +
> +# define MAP_TRYFIXED (MAP_FIXED | MAP_EXCL) /* BSD name. */
> +# define MAP_FIXED_NOREPLACE (MAP_FIXED | MAP_EXCL) /* Linux name. */
> #endif /* __USE_GNU */
> diff --git a/sysdeps/mach/hurd/mmap.c b/sysdeps/mach/hurd/mmap.c
> index 33672cf6..20264a77 100644
> --- a/sysdeps/mach/hurd/mmap.c
> +++ b/sysdeps/mach/hurd/mmap.c
> @@ -46,6 +46,9 @@ __mmap (void *addr, size_t len, int prot, int flags, int
> fd, off_t offset)
> if ((mapaddr & (__vm_page_size - 1)) || (offset & (__vm_page_size - 1)))
> return (void *) (long int) __hurd_fail (EINVAL);
>
> + if ((flags & MAP_EXCL) && ! (flags & MAP_FIXED))
> + return (void *) (long int) __hurd_fail (EINVAL);
> +
> vmprot = VM_PROT_NONE;
> if (prot & PROT_READ)
> vmprot |= VM_PROT_READ;
> @@ -156,15 +159,20 @@ __mmap (void *addr, size_t len, int prot, int flags,
> int fd, off_t offset)
> {
> if (err == KERN_NO_SPACE)
> {
> - /* XXX this is not atomic as it is in unix! */
> - /* The region is already allocated; deallocate it first. */
> - err = __vm_deallocate (__mach_task_self (), mapaddr, len);
> - if (! err)
> - err = __vm_map (__mach_task_self (),
> - &mapaddr, (vm_size_t) len, mask,
> - 0, memobj, (vm_offset_t) offset,
> - copy, vmprot, max_vmprot,
> - copy ? VM_INHERIT_COPY : VM_INHERIT_SHARE);
> + if (flags & MAP_EXCL)
> + err = EEXIST;
> + else
> + {
> + /* The region is already allocated; deallocate it first. */
> + /* XXX this is not atomic as it is in unix! */
> + err = __vm_deallocate (__mach_task_self (), mapaddr, len);
> + if (! err)
> + err = __vm_map (__mach_task_self (),
> + &mapaddr, (vm_size_t) len, mask,
> + 0, memobj, (vm_offset_t) offset,
> + copy, vmprot, max_vmprot,
> + copy ? VM_INHERIT_COPY : VM_INHERIT_SHARE);
> + }
> }
> }
> else
> --
> 2.41.0
>
--
Samuel
---
Pour une évaluation indépendante, transparente et rigoureuse !
Je soutiens la Commission d'Évaluation de l'Inria.
