amd64/127276: ldd invokes linux yes

[Dominic Fandrey]

John Baldwin wrote:
> On Sunday 14 September 2008 10:33:47 am Dominic Fandrey wrote:
>> John Baldwin wrote:
>>> FreeBSD binaries from various releases have been branded in different 
> ways.  I 
>>> would consider it more of a user error to run ldd on a Linux binary. :)  
> You 
>>> could maybe add a "IMPLEMENTATION NOTES" section to the manpage that 
> explains 
>>> how it works and why it will execute any binary using a different runtime 
>>> linker.
>>>
>> Well, documenting it is much better than the current state. Though in my
>> opinion it doesn't matter to the user how it works, but what one expects the
>> program to do. And the current behaviour is not what I expected.
>>
>> Would you instead accept a patch from me that does a compatibility check and
>> bails out if the binary does not use the FreeBSD linker?
> 
> It can be hard to determine that.  What happens is that each ELF binary 
> includes a path to its interpreter (i.e. the runtime linker).  For FreeBSD 
> binaries this can be either /usr/libexec/ld-elf.so.1 or /libexec/ld-elf.so.1 
> or the a.out paths (/usr/libexec/ld.so.1 I think).  In the kernel, ABI 
> modules hook into exec and when they see a binary they can handle, they can 
> choose to overwrite the interpreter path to point it to somewhere else (e.g. 
> the Linux ABI uses a path under /compat/linux instead, and the 'freebsd32' 
> ABI on amd64 uses /libexec/ld-elf32.so.1).  Any ELF binary that uses one of 
> the two ld-elf.so.1 (or ld-elf32.so.1) paths will use the FreeBSD runtime 
> linker, regardless of which "OS" the binary is targeted for.  You could maybe 
> hardcode the list of interpreter strings to check for, but that wouldn't be 
> completely foolproof.  You could have an ABI that is fine with using the 
> FreeBSD linker even though its native "OS" uses a different interpreter path 
> (though that is unlikely).  In that case the kernel module would be rewriting 
> the interpreter path to be the FreeBSD ld-elf, but ldd would have no clue.
> 

I wanted to look into the file and readelf tools to check how they do it.
I'm using readelf to avoid calling ldd when inappropriate and I think it's
reliable, so there aught to be a way.

I'm kinda stressed, but I'm going to look into it in a couple of weeks. If
it doesn't pay off. Well, I'm all for the implementation notes.