Re: Possible bug; code-block vanishes when adding one assembler-instruction.

[Richard Sandiford <rdsandiford at googlemail dot com>]

Jens Bauer <jens-lists@gpio.dk> writes:
>> The difference comes from whether the alignment in:
>> 
>>           . = ALIGN (4);
>>           _efastcode = . ;
>> 
>> actually does anything or not.  If the address is already correctly
>> aligned, because the size of the input .fastcode section is divisible by 4,
>> the section stays unallocatable.  If the assignment to "." increases the
>> address (inserts padding) then the section becomes allocatable.
>
> OK. I would say that the correct behaviour in this case, would be to not
> include anything at all; how silly it may sound. ;)

I agree.  The linker ought to do the same thing regardless of whether
an assignment inserts zero or nonzero padding.  I'm afraid of doing
anything for compatibility reasons though.

> -But... I think the assembler could be improved slightly here.
>
> It isn't often that the .section directive is used in an assembly-source file.
> -So if it's used, the user _really_wants_ the output, I believe.
> Especially if the user created a .section .freddy, which was followed by
> assembly-language instructions (or any indication of using instructions
> inside the section), in my opinion, it would be right to mark it as "ax"
>
> ...Or the assembler could perhaps mark bare .section .foo as
> "ax"/progbits by default; I guess it would be possible to create a plain
> .section .foo, by just using
> 	.section	.foo,"",@nobits

Backwards compatibility would be a problem here too though.
Particularly since marking something as executable when we aren't sure
isn't always the safe option, especially from a security point of view.

E.g. things like .gnu.warning sections are like your .foo above,
but tend not to have the '"", @nobits'.  Lines like:

      .section .rodata

are also common -- at least from a quick scan of glibc -- and .rodata
shouldn't be executable.  (.rodata itself is treated specially by gas
and gets the allocatable flag, but maybe people are following this
example for other sections that gas doesn't know about, just like
.fastcode vs .text in your case.)

> ...Or perhaps even more foolproof... Mark section with all flags (I
> know, this may be silly) ?
> What I'm trying to say is that.. Either put up an error and complain or
> stay on the safe side. ;)

Complaining is probably better than trying to second-guess.
I agree that warning when inserting code into an implicitly
non-executable section might make sense.  Unfortunately,
because of the way gas is structured, it might be quite
tricky to implement...

Thanks,
Richard