/dev/fd/ support

[TejaswineeL]

Description of the problem

I am getting below error in gramine when trying to execute process substitution with gramine. Named pipe file /dev/fd/63 is not getting detetcted by gramine:

diff: /dev/fd/63: No such file or directory

Process substitution is a form of inter-process communication that allows the input or output of a command to appear as a file. This allows programs that normally only accept files to directly read from or write to another program.
Reference: https://en.wikipedia.org/wiki/Process_substitution

Steps to reproduce

gramine 1.4

1. Navigate to CI-Examples/bash
2. Create 2 dummpy text files (here file1 and file2)
3. Run the command gramine-direct bash -c 'diff file2 <(sort file1)'

(Here, result of sort file1 is stored to named pipe file /dev/fd/63. Now this run time generated (named pipe) file acts as argument for diff command
so the given command becomes diff file2 /dev/fd/63)

Expected results

we should see the unmatched(uncommon) text out of the 2 files (as diff stands for differenciation)

Actual results

diff: /dev/fd/63: No such file or directory
process_substitution_failure_log.txt

Gramine commit hash

gramine 1.4-aefd5d0875c4fdf76edd67f5043952bdd32b0f7a

[mkow]

This doesn't work because bash is using /dev/fd/ which isn't implemented at all in Gramine.

[dimakuv]

This doesn't work because bash is using /dev/fd/ which isn't implemented at all in Gramine.

Which is actually bad. I think we need to implement these pseudo-dirs to operate on FDs sooner than later...

[mkow]

Should be easy, we already have them in /proc/self/fd/.

[kailun-qin]

Should be easy, we already have them in /proc/self/fd/.

There's currently no dentry associated w/ the paths of the pipes created during this process substitution. So even if we have /dev/fd/ (linking to /proc/self/fd/), I guess we may not be able to enable this exmple (gramine-xxx bash -c 'diff file2 <(sort file1)') which can be implemented via e.g., stat() underneath.

[mkow]

There's currently no dentry associated w/ the paths of the pipes created during this process substitution

First, let's switch to Linux-level terms, "process substitution" is a Bash-specific feature ;) So, you're talking here about support for named pipes? (I think we support them?) Or that we don't create /proc/self/fd/ entry after creation of anonymous pipes? (pipe())

can be implemented via e.g., stat() underneath

Not sure what do you mean here.

[kailun-qin]

Or that we don't create /proc/self/fd/ entry after creation of anonymous pipes? (pipe())

Yes, this is what I meant. I.e., if we do a stat() on /proc/self/fd/xxx right after pipe(), I don't think stat() will succeed. Since the example may have similar flows, it might not work.

[mkow]

Hmm, it seem to work only partially, we create an entry there, but without a proper dentry. I guess we'd need to add it to make it functional: (this is under gramine-direct)

bash-5.0# ls -la <(echo 123) /proc/self/fd/63
ls: cannot access '/dev/fd/63': No such file or directory
ls: /proc/self/fd/63: Function not implemented
lrwxrwxrwx 1 0 0 0 Jan  1  1970 /proc/self/fd/63 -> pipe:[?]

bash-5.0# cat <(echo 123) /proc/self/fd/63
cat: /dev/fd/63: No such file or directory
cat: /proc/self/fd/63: No such file or directory