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Support mount_setattr(2) attributes (mostly for recursively-readonly mounts) #1090

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Support mount_setattr(2) attributes (mostly for recursively-readonly mounts) #1090

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35 changes: 35 additions & 0 deletions config.md
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Expand Up @@ -139,6 +139,41 @@ For POSIX platforms the `mounts` structure has the following fields:
]
```

### <a name="configLinuxMounts" />Linux Mounts

For Linux, the `mounts` structure mounts MAY have the following additional fields:
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* **`attr`** (object, OPTIONAL) specifies `mount_setattr(2)` attributes. Requires kernel 5.12 or later.
The runtime MUST [generate an error](runtime.md#errors) for any values which cannot be mapped to a relevant kernel interface.
* **`flags`** (array of strings, OPTIONAL) specifies the `flags` argument of `mount_setattr(2)` syscall.
e.g., "AT_RECURSIVE"
* **`attr_set`** (array of strings, OPTIONAL) specifies the `attr_set` field of `mount_attr` struct.
e.g., "MOUNT_ATTR_RDONLY"
* **`attr_clr`** (array of strings, OPTIONAL) specifies the `attr_clr` field of `mount_attr` struct.
* **`propagation`** (string, OPTIONAL) specifies the `propagation` field of `mount_attr` struct.
See [Rootfs Mount Propagation](./config-linux.md#configLinuxRootfsMountPropagation) for the string format.

<!-- TODO: consider supporting mount_attr.userns_fd -->

### Example

```json
"mounts": [
{
"destination": "/ro",
"type": "none",
"source": "/volumes/ro",
"options": ["rbind"],
"attr": {
"flags": ["AT_RECURSIVE"],
"attr_set": ["MOUNT_ATTR_RDONLY"],
"attr_clr": ["MOUNT_ATTR_NOEXEC"],
"propagation": "private"
}
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@thaJeztah thaJeztah Aug 18, 2021

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I'm a bit concerned about how this is just a 1:1 mapping of mount_setattr (although I realise we already crossed that path on many other options). The advantage is that it's flexible, but it we only need it for the recursively-readonly, I'm wondering if it would be good to design around that case, making it somewhat more less verbose?

It makes it a bit confusing in some areas, e.g., options would take (r)private, ro and noexec, but attr takes MOUNT_ATTR_RDONLY and MOUNT_ATTR_NOEXEC (in addition to using a "diff" of attributes to add and to remove).

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@AkihiroSuda AkihiroSuda Aug 18, 2021 •
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The advantage is that it's flexible, but it we only need it for the recursively-readonly, I'm wondering if it would be good to design around that case, making it somewhat more less verbose?

I don't think this is going to be recursively-readonly only, though we can prioritize recursively-readonly over other potential usecases

It makes it a bit confusing in some areas, e.g., options would take (r)private, ro and noexec, but attr takes MOUNT_ATTR_RDONLY and MOUNT_ATTR_NOEXEC (in addition to using a "diff" of attributes to add and to remove).

MOUNT_ATTR_XXX form corresponds to RLIMIT_XXX form that has already existed.

}
]
```

## <a name="configProcess" />Process

**`process`** (object, OPTIONAL) specifies the container process.
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20 changes: 20 additions & 0 deletions schema/defs.json
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Expand Up @@ -141,12 +141,32 @@
},
"type": {
"type": "string"
},
"attr": {
"$ref": "#/definitions/LinuxMountAttr"
}
},
"required": [
"destination"
]
},
"LinuxMountAttr": {
"type": "object",
"properties": {
"flags": {
"$ref": "#/definitions/ArrayOfStrings"
},
"attr_set": {
"$ref": "#/definitions/ArrayOfStrings"
},
"attr_clr": {
"$ref": "#/definitions/ArrayOfStrings"
},
"propagation": {
"type": "string"
}
}
},
"ociVersion": {
"description": "The version of Open Container Initiative Runtime Specification that the document complies with",
"type": "string"
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16 changes: 16 additions & 0 deletions specs-go/config.go
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Expand Up @@ -117,6 +117,22 @@ type Mount struct {
Source string `json:"source,omitempty"`
// Options are fstab style mount options.
Options []string `json:"options,omitempty"`
// Attr specifies mount_setattr(2) attributes.
Attr *LinuxMountAttr `json:"attr,omitempty"`
}

// LinuxMountAttr specifies mount_setattr(2) attributes.
// Requires Linux kernel 5.12 or later.
type LinuxMountAttr struct {
// Flags specifies the flags argument of mount_setattr(2) syscall. e.g., "AT_RECURSIVE"
Flags []string `json:"flags,omitempty"`
// AttrSet specifies the attr_set field of mount_attr struct. e.g., "MOUNT_ATTR_RDONLY"
AttrSet []string `json:"attr_set,omitempty"`
// AttrClr specifies the attr_clr field of mount_attr struct.
AttrClr []string `json:"attr_clr,omitempty"`
// Propagation specifies the propagation field of mount_attr struct.
// Format corresponds to Linux.RootfsPropagation .
Propagation string `json:"propagation,omitempty"`
}

// Hook specifies a command that is run at a particular event in the lifecycle of a container
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