[PATCH proxmox] shared-memory: flush contents of newly created mmap file

[PATCH proxmox] shared-memory: flush contents of newly created mmap file

[Christian Ebner]

The man page for mmap [0] states, that mappings create with
`MAP_SHARED` are not carried to the underlying file directly.

To avoid possible races between creating and initializing a new
mmapped file and opening while checking the header of this file,
msync the contents after init but before persisting the shared
memory backed file.

[0] https://man7.org/linux/man-pages/man2/mmap.2.html

Fixes: https://forum.proxmox.com/threads/182948/
Signed-off-by: Christian Ebner <c.ebner@proxmox.com>
---
 proxmox-shared-memory/src/lib.rs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/proxmox-shared-memory/src/lib.rs b/proxmox-shared-memory/src/lib.rs
index 495812ef..4838ab42 100644
--- a/proxmox-shared-memory/src/lib.rs
+++ b/proxmox-shared-memory/src/lib.rs
@@ -190,6 +190,10 @@ impl<T: Sized + Init> SharedMemory<T> {
 
         let mmap = mmap_file(&mut file, true)?;
 
+        if skip_tmpfs_check {
+            mmap.msync(MsFlags::MS_SYNC)?;
+        }
+
         let res = {
             let path = CString::new(path.as_os_str().as_bytes())?;
             Errno::result(unsafe {
-- 
2.47.3

applied: [PATCH proxmox] shared-memory: flush contents of newly created mmap file

[Fabian Grünbichler]

just FTR, this one got applied already in
acd33a15c29cfe617ce5ecc69c78f692b2f20a8b

On April 22, 2026 9:33 am, Christian Ebner wrote:
> The man page for mmap [0] states, that mappings create with
> `MAP_SHARED` are not carried to the underlying file directly.
> 
> To avoid possible races between creating and initializing a new
> mmapped file and opening while checking the header of this file,
> msync the contents after init but before persisting the shared
> memory backed file.
> 
> [0] https://man7.org/linux/man-pages/man2/mmap.2.html
> 
> Fixes: https://forum.proxmox.com/threads/182948/
> Signed-off-by: Christian Ebner <c.ebner@proxmox.com>
> ---
>  proxmox-shared-memory/src/lib.rs | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/proxmox-shared-memory/src/lib.rs b/proxmox-shared-memory/src/lib.rs
> index 495812ef..4838ab42 100644
> --- a/proxmox-shared-memory/src/lib.rs
> +++ b/proxmox-shared-memory/src/lib.rs
> @@ -190,6 +190,10 @@ impl<T: Sized + Init> SharedMemory<T> {
>  
>          let mmap = mmap_file(&mut file, true)?;
>  
> +        if skip_tmpfs_check {
> +            mmap.msync(MsFlags::MS_SYNC)?;
> +        }
> +
>          let res = {
>              let path = CString::new(path.as_os_str().as_bytes())?;
>              Errno::result(unsafe {
> -- 
> 2.47.3
> 
> 
> 
> 
> 
> 

applied: [PATCH proxmox] shared-memory: flush contents of newly created mmap file

[Thomas Lamprecht]

On Wed, 22 Apr 2026 09:33:51 +0200, Christian Ebner wrote:
> The man page for mmap [0] states, that mappings create with
> `MAP_SHARED` are not carried to the underlying file directly.
> 
> To avoid possible races between creating and initializing a new
> mmapped file and opening while checking the header of this file,
> msync the contents after init but before persisting the shared
> memory backed file.
> 
> [...]

Applied, thanks!

[1/1] shared-memory: flush contents of newly created mmap file
      commit: acd33a15c29cfe617ce5ecc69c78f692b2f20a8b