* [9fans] fd and /srv filesystem
@ 2023-10-04 10:29 Chris McGee
2023-10-04 12:20 ` hiro
2023-10-04 16:06 ` ori
0 siblings, 2 replies; 5+ messages in thread
From: Chris McGee @ 2023-10-04 10:29 UTC (permalink / raw)
To: Fans of the OS Plan 9 from Bell Labs
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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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* Re: [9fans] fd and /srv filesystem
2023-10-04 10:29 [9fans] fd and /srv filesystem Chris McGee
@ 2023-10-04 12:20 ` hiro
2023-10-04 12:25 ` hiro
2023-10-04 16:06 ` ori
1 sibling, 1 reply; 5+ messages in thread
From: hiro @ 2023-10-04 12:20 UTC (permalink / raw)
To: 9fans
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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* Re: [9fans] fd and /srv filesystem
2023-10-04 12:20 ` hiro
@ 2023-10-04 12:25 ` hiro
0 siblings, 0 replies; 5+ messages in thread
From: hiro @ 2023-10-04 12:25 UTC (permalink / raw)
To: 9fans
btw it's very common on unix to share FDs in multi-threaded programs.
and all the pain resulting from un-synchronised FD access is available
as expected :)
On 10/4/23, hiro <23hiro@gmail.com> wrote:
> 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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* Re: [9fans] fd and /srv filesystem
2023-10-04 10:29 [9fans] fd and /srv filesystem Chris McGee
2023-10-04 12:20 ` hiro
@ 2023-10-04 16:06 ` ori
2023-10-04 23:42 ` Chris McGee
1 sibling, 1 reply; 5+ messages in thread
From: ori @ 2023-10-04 16:06 UTC (permalink / raw)
To: 9fans
Quoth Chris McGee <newton688@gmail.com>:
> 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
>
it's all just Chans in the kernel; devsrv just provides
a way of giving an open chan a name in the namespace.
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* Re: [9fans] fd and /srv filesystem
2023-10-04 16:06 ` ori
@ 2023-10-04 23:42 ` Chris McGee
0 siblings, 0 replies; 5+ messages in thread
From: Chris McGee @ 2023-10-04 23:42 UTC (permalink / raw)
To: 9fans
Thanks all for the explanations. I think I understand better now.
Chris
> On Oct 4, 2023, at 12:06 PM, ori@eigenstate.org wrote:
>
> Quoth Chris McGee <newton688@gmail.com>:
>> 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
>>
>
> it's all just Chans in the kernel; devsrv just provides
> a way of giving an open chan a name in the namespace.
>
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2023-10-04 10:29 [9fans] fd and /srv filesystem Chris McGee
2023-10-04 12:20 ` hiro
2023-10-04 12:25 ` hiro
2023-10-04 16:06 ` ori
2023-10-04 23:42 ` Chris McGee
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