Re: A running list of questions from "porting" Slackware to musl

[Rich Felker <dalias@libc.org>]

On Tue, Sep 30, 2014 at 08:32:16AM -0700, Isaac Dunham wrote:
> On Tue, Sep 30, 2014 at 06:13:37PM +0530, Weldon Goree wrote:
> > Hi all,
> > 
> > I added the quotation marks of shame because it's not a "port" in a real
> > sense. But still: I've had this side project[1] for a while of porting
> > Slackware to use Musl and it's Nearly There (tm), but I was hoping for some
> > advice on some persistent irritations I have. (Sorry for the length.)
> <snip> 
> > 6. Stack protection. This one really puzzles me. Stack protection is as
> > alien to glibc as it is to musl, but I keep running into this. 90% of the
> > problems can be avoided with adding -fno-stack-protector appropriately, but
> > libtool is very "helpful" on matters like this and seems to find a way to
> > put it back. I've actually not found an unworkable problem yet (though
> > several very annoying ones); I guess I'm just curious what the real state of
> > ssp on musl is (I'm not a fan of the concept, personally, but I know a lot
> > of people are), and whether there's a general solution to just telling
> > software to trust the ****ing stack.
> 
> You need a "libssp_nonshared.a" containing a function named
> __stack_chk_fail_local, which need only call __stack_chk_fail.
> No idea why, but this cannot be in a shared libary.

When gcc generates the canary-check code, on failure it normally
calls/jumps to __stack_chk_fail. But for shared libraries, that call
would go to a thunk in the library's PLT, which depends on the GOT
register being initialized (actually this varies by arch; x86_64
doesn't need it). In order to avoid (expensive) loading of the GOT
register in every function just as a contingency in case
__stack_chk_fail needs to be called, for position-independent code GCC
generates a call to __stack_chk_fail_local instead. This is a hidden
function (and necessarily exists within the same .so) so the call
doesn't have to go through the PLT; it's just a straight relative
call/jump instruction. __stack_chk_fail_local is then responsible for
loading the GOT register and calling __stack_chk_fail.

Rich