[PATCH] Using MAP_SHARE_VALIDATE in mmap without fd

[PATCH] Using MAP_SHARE_VALIDATE in mmap without fd

[Nils Hartmann]

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?
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.

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;
-- 
2.39.2

Re: [PATCH] Using MAP_SHARE_VALIDATE in mmap without fd

[David Hildenbrand]

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

Re: [PATCH] Using MAP_SHARE_VALIDATE in mmap without fd

[Nils Hartmann]

On Thu, Mar 16, 2023 at 04:28:21PM +0100, David Hildenbrand wrote:
> On 10.03.23 18:16, Nils Hartmann wrote:
> > Hey,
> > 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".

Yup thats what I meant.

> 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.

Yeah I heard about it being introduced with MAP_SYNC.
But since the manpage from mmap specifically says:
'MAP_SHARED_VALIDATE provides the same behaviour as MAP_SHARED',
I didn't think it would make a difference

> 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.

The only reason I can come up with, is naming consistency.
But it's really a non-issue und updating the man page is definitly
the saner option.

> --
> Thanks,
>
> David / dhildenb
>

Best Regards,

Nils