Re: Question about setting argv[0] when manually using dynamic linker

[John Regan <saxindustries@gmail.com>]

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On Wed, May 17, 2017 at 2:07 PM, <u-uy74@aetey.se> wrote:

> On Wed, May 17, 2017 at 12:24:28PM -0400, Rich Felker wrote:
> > > >>> On Tue, May 16, 2017 at 08:38:56PM -0400, John Regan wrote:
> > > >>> > Hi there - I was wondering if it's possible to somehow set
> argv[0] when
> > > >>> > calling the dynamic linker to load a program.
>
> > > >>> Set argv[0] to whatever you need when you exec*() the dynamic
> loader,
> > > >>> which is straightforward with a binary wrapper (not with a shell).
> > > >>>
> > > >>> A binary wrapper also adds less overhead then going through a
> shell.
>
> > > >>> Rune
>
> > a completely reasonable and recommended way for deploying dynamic
> > linked apps in a self-contained way that doesn't depend on musl libc
> > on the host. Unfortunately there's no way to set argv[0] like you want
>
> We do deploy dynamic linked apps without any dependencies on the libraries
> on the host. It works just fine with musl-as-it-is, including the
> questionably designed applications like busybox and gcc who
> analyze argv[0].
>
> > at this time. Perhaps adding an option like --argv0=foo would be
> > appropriate.
>
> What would be the justification for adding the supporting code (to every
> instance of the dynamic loader)?
>
> It looks like --argv0=foo is meant to overcome a specific limitation in
> bourne shell, in a specific context where the task can be solved easily
> and generally better without involving the bourne shell in the first hand.
>
> I would like to see an example of a situation where a wrapper in C (or
> any language allowing setting of argv[0]) is less appropriate?
>
> If one really has a reason to express the wrapper in sh, a one-liner in
> C and an extra exec from the shell (much cheaper than starting the
> shell itself was) is sufficient to make it work.
>
> Rune
>
> Hi Rune - would you mind sharing some tips on doing that?

I wrote and compiled a short program that just dumps the elements in argv,
then a wrapper program that figures out the needed paths for libc, real
binary, etc, but it seems like argv[0] gets reset by the dynamic loader.

I'm calling execve with the path to the libc.so, and argv is somenthing
like:

argv[0] - desired process name
argv[1] - full path to the real binary
argv[2...] arguments

The 'real' binary is loaded and ran, but winds up printing out:

argv[0] - full path to the real binary
argv[1...] arguments

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