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proposal: os: add methods to safely convert between files and roots #70370

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Jorropo opened this issue Nov 15, 2024 · 3 comments
Open

proposal: os: add methods to safely convert between files and roots #70370

Jorropo opened this issue Nov 15, 2024 · 3 comments
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@Jorropo
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Jorropo commented Nov 15, 2024 •
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Proposal Details

I have code which opens both files and directories as files to perform various actions such as fetching and comparing generations (I need to upstream to linux once I write tests) or other metadata with SyscallConn.

Trying to use #67002 to make it less conservative with some TOCTOU edge cases prove challenging since *os.Root does not provide needed SyscallConn to fetch the generation of directories and more architecturally the current codepath is to open everything as a *os.File and then use statx to decide what to do next.

package os

// AsRoot open the File as a Root if it is a directory otherwise it errors.
// The Root is returned with a new lifetime such that each need to be closed independently.
func (*File) AsRoot() (*Root, error)

// AsFile opens the Root as a File.
// The File is returned with a new lifetime such that each need to be closed independently.
func (*Root) AsFile() (*File, error)
@gopherbot gopherbot added this to the Proposal milestone Nov 15, 2024
@Jorropo
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Jorropo commented Nov 15, 2024

I discussed this with @neild already and:
I think it might be possible to convert *os.File → *os.Root by using SyscallConn then calling os.OpenRoot with fmt.Sprintf("/proc/self/fd/%d") on Linux.
Not portable tho.

He pointed out *os.Root → *os.File should be possible by calling (*os.Root).Open(root, ".").

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Jorropo commented Nov 15, 2024 •
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I am not sure if lifetimes should be independent or not.
I guess from a point of implementing in the std this is the easiest, but as a consumer I would sometime want the opposite.

@ianlancetaylor ianlancetaylor moved this to Incoming in Proposals Nov 15, 2024
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neild commented Nov 19, 2024

I would not want to try to have a Root and a File as two views of the same file descriptor. (In some cases, Roots don't even have a file descriptor.)

You can go from a Root to a File with root.Open("."), at the expense of creating a new FD.

Going from a File to a Root is not currently possible. I can imagine a method on File which lets you do so:

// Root returns a root rooted at the location of f, which must be a directory file.
func (f *File) Root() (*Root, error)

I don't know if there's a need for that operation; motivating use cases would be useful.

It might be reasonable to add a Root.SyscallConn method to give access to a root's FD, with the caveat that this method would return an error on platforms where a root doesn't have an FD (GOOS=js, GOOS=plan9). But perhaps that isn't necessary, and root.Open(".") is sufficient.

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