rtld: introduce PRELOAD_FDS

[oshogbo]

Summary

The new PRELOAD_FDS variable accepts a list of file descriptors
that should be loaded into the process.

This may be used to optimize a loading process - in the case when
we already have a file descriptor to the library; we don't have
to look into multiple PATH to find it.

It may also be used in capability mode to load a single additional
library without the need to open a directory that contains it.

The last use of this functionality t may be a race-free method
of loading libraries.

[oshogbo]

This comment was removed by oshogbo.

[markj]

libexec/rtld-elf/rtld.c

2579

So here we dup the passed fd to set CLOEXEC. Does the original fd get closed somehow?

[oshogbo]

libexec/rtld-elf/rtld.c

2579

No, we do not.
I did the same think like in case of LD_LIBARARY_PATH_FDS.
We don't close anywhere the directories passed.
Now I think of that the consumer should have decision what to do with those fds (in both cases) right?

[kib]

libexec/rtld-elf/rtld.c

2491

return (-1);

You can stage a pre-patch changing other returns in this function, if you prefer common style. Or better, convert the function to style (mostly indents).

[markj]

libexec/rtld-elf/rtld.c

2579

I misread the code, I didn't see that fd is closed below.