Re: [9fans] fd and /srv filesystem

[hiro <23hiro@gmail.com>]

file descriptors describe to the kernel which of the files you
previously open()'ed (a syscall) you want to operator on.

it's not about security: if you want to operate on a file that another
process might have opened before, you have to be careful that the
other process isn't writing to the same location in the file at the
same time. the kernel also keeps offsets for you.

if you share FDs between multiple processes you might want some
synchronisation like locking.

On 10/4/23, Chris McGee <newton688@gmail.com> wrote:
> Hi All,
> 
> I was thinking about file descriptors in the context of Plan 9. On Unix an
> fd is generally only usable by the current process, and child ones through
> a fork with some special incantation if one wants to communicate one over a
> domain socket. This is possibly for security reasons, avoiding other users'
> processes from trying to guess the fd of a critical file.
> 
> It's common practice in Plan 9 to post an fd (sometimes via a pipe) from
> one process to the /srv filesystem so that others can discover it and open
> a comms channel. Does the kernel transform the fd into something when
> posted to /srv so that it can be consumed by any other process in the
> system?
> 
> Thanks,
> Chris

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