rtld: properly handle update of several vars in rtld_set_var()

[kib]

Summary

Besides setting the value in the array of the values, rtld sometimes
needs to recalculate some internal control variable for the change to
take effect.  Allow the variable description to supply a method called
on the update.  Lock the function with the bind lock for safe operation.

Mark several variables as allowed for update, since the on_update method
is provided for them.  The list is LD_BIND_NOW, LD_BIND_NOT,
LD_LIBMAP_DISABLE, LD_LOADFLTR.

[markj]

libexec/rtld-elf/rtld.c

6737

Is this missing the on_update call?

[kib]

Do call on_update()

[markj]

libexec/rtld-elf/rtld.c

6626

Does ld_debug need to be a global variable?

6651

Shouldn't we set it to false if the value is NULL?

6663

Should ld_bind_not be updated here if lvd->val is NULL? That's what happens during image activation time.

6688

Should dangerous_ld_env be recalculated if this variable (and some others) are toggled?

[kib]

libexec/rtld-elf/rtld.c

6626

I removed ld_debug altogether.

6651

I am not sure, but lets do it.

[kib]

libexec/rtld-elf/rtld.c

6688

I decided to never reset dangerous_ld_env back to false is it ever set to true.

[kib]

Handle recalculations second-order effects.

[markj]

libexec/rtld-elf/rtld.c

6626

|= looks wrong.

6696

Use rtld_recalc_bind_not()?

[kib]

libexec/rtld-elf/rtld.c

6626

Could you please explain why? I want to never flip it true->false.

[kib]

More uses for rtld_recalc_bind_not().

[markj]

libexec/rtld-elf/rtld.c

6626

I wasn't sure why it should never flip true -> false, but after thinking some more it seems reasonable if one reads the description of the dangerous_ld_env variable. Maybe some short comment would be good to have.

[kib]

Note that dangerous_ld_env is never reset back to false.

[emaste]

libexec/rtld-elf/rtld.c

392–393

This means that some variables e.g. BIND_NOW can now be updated, right? I think those should be noted in the commit message too.

[kib]

Update the commit message with the list of vars allowed in the commit.
Provide the whole list of changeable vars in rtld_get_var(3).

[kib]

Add test from PR 294054