Re: [PATCH] redir: Use memfd_create instead of pipe

[Jilles Tjoelker <jilles@xxxxxxxx>]

On Sun, Apr 14, 2024 at 03:51:37PM +0800, Herbert Xu wrote:
> Use memfd_create(2) instead of pipe(2).  With pipe(2), a fork
> is required if the amount of data to be written exceeds the pipe
> size.  This is not the case with memfd_create.

Since a memfd does not behave identically to a pipe, this should be
tested carefully. A memfd does not behave identically to a regular file
either. It may affect programs started from the shell that read
here-documents.

Using pipe or memfd conditionally based on the length of the
here-document and whether memfd_create(2) fails transiently might cause
even more obscure issues.

I suggest using either a pipe for all here-documents or a memfd for all
here-documents. The shell should fall back from memfd_create(2) to
pipe(2) only if memfd_create(2) is not supported or fails for a
persistent reason like [ENOSYS] or [EPERM]; in this case, the shell
should remember the failure and immediately use pipe(2) for subsequent
here-documents.

The dup/close dance with the memfd will look silly in syscall traces,
although it is functionally fine.

-- 
Jilles Tjoelker