linux-kernel - Re: [PATCH] Using MAP_SHARE

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Message-ID: <e6b72d88-3011-c284-f53a-71431065b991@redhat.com>
Date:   Thu, 16 Mar 2023 16:28:21 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Nils Hartmann <nils1hartmann@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Using MAP_SHARE_VALIDATE in mmap without fd

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@...il.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