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SubjectRe: [PATCH] Using MAP_SHARE_VALIDATE in mmap without fd
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On 10.03.23 18:16, Nils Hartmann wrote:
> Hey,
> I have a rather simple question about the MAP_SHARED_VALIDATE flag in mmap.
> When used without a file pointer, EINVAL is returned. Is there a reason for this?

You mean, using it with shared anonymous memory? (MAP_SHARED|MAP_ANON)

I assume you mean "file descriptor" not "file pointer".

> I researched a bit but could not find anything. I attached a simple patch that adds MAP_SHARE_VALIDATE to the flags switch and checks for invalid flags.
>

The only reason MAP_SHARED_VALIDATE was introduced was due to MAP_SYNC,
only required for DAX. DAX does not apply to shared anonymous memory.

I guess nobody cared/cares.

Question is if we want to update the implementation (there has to be a
good reason IMHO) or simply update the man page, stating that
MAP_SHARED_VALIDATE is not supported for MAP_ANON.

> Signed-off-by: Nils Hartmann <nils1hartmann@gmail.com>
> ---
> mm/mmap.c | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 740b54be3..fd7db51af 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1196,6 +1196,7 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
> {
> struct mm_struct *mm = current->mm;
> vm_flags_t vm_flags;
> + unsigned long flags_mask;
> int pkey = 0;
>
> validate_mm(mm);
> @@ -1266,14 +1267,14 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
> if (mlock_future_check(mm, vm_flags, len))
> return -EAGAIN;
>
> + flags_mask = LEGACY_MAP_MASK;
> if (file) {
> struct inode *inode = file_inode(file);
> - unsigned long flags_mask;
>
> if (!file_mmap_ok(file, inode, pgoff, len))
> return -EOVERFLOW;
>
> - flags_mask = LEGACY_MAP_MASK | file->f_op->mmap_supported_flags;
> + flags_mask |= file->f_op->mmap_supported_flags;
>
> switch (flags & MAP_TYPE) {
> case MAP_SHARED:
> @@ -1327,6 +1328,10 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
> }
> } else {
> switch (flags & MAP_TYPE) {
> + case MAP_SHARED_VALIDATE:
> + if (flags & ~flags_mask)
> + return -EOPNOTSUPP;
> + fallthrough;
> case MAP_SHARED:
> if (vm_flags & (VM_GROWSDOWN|VM_GROWSUP))
> return -EINVAL;

--
Thanks,

David / dhildenb

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