Re: _start(ldso) & _start(crt)

[Rich Felker <dalias@libc.org>]

On Mon, Apr 21, 2014 at 03:41:23PM -0400, writeonce@midipix.org wrote:
> Greetings,
> 
> When running a dynamically linked program, the general control flow is
> 
>     _start(ldso/{arch}/start.s) --> __dynlink --> _start(crt) -->
> __libc_start_main --> main()
> 
> When asking gdb to set up a breakpoint at _start (b _start), gdb's
> confirmation message shows the address of _start(crt), but it then
> breaks at _start(ldso), which is actually what I want.  It then
> later also breaks at _start(crt).
> 
> If gdb decided to only break at the executable's _start, then I
> believe there would be no way to request a break at the
> interpreter's (ldso)  _start routine, since the address of ldso's
> _start is not known prior to execution.
> 
> I am not sure whether this is a good-enough motivation to rename
> ldso's start to _ldso_start (and set libc.so's entry point
> accordingly), or whether we could simply count on gdb and other
> debuggers to continue to do the "right thing" by breaking at the
> earlier _start of the two.  For one thing, setting a breakpoint at
> __dynlink provides pretty much the same debugging ability as with
> _start(ldso), and does not require any changes.  Any thoughts?

The motivation for the name _start in the dynamic linker was
presumably that it gets used by default as the entry point, but we
already have -Wl,-e,_start anyway, so I don't see any compelling
reason it couldn't be renamed. Does anyone else object to changing it?

Rich