Re: dynamic linker command line invocation

[N Jain <jain61@gmail.com>]

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Hi Rich,

Thanks I am able to get my dynamic linker loaded into memory but facing
issue during application load at __dls3 function.
I am getting following print from MUSL -

"musl libc
Version 1.1.10
Dynamic Program Loader
Usage: hello.elf [options] [--] pathname--2--"

Not sure why my application is not getting loaded in below code __dls3
function ?

char *ldname = argv[0];
size_t l = strlen(ldname);

*ldname is the application elf which I want to load "hello.elf"*
if (l >= 3 && !strcmp(ldname+l-3, "ldd")) ldd_mode = 1;
*I am not sure why "ldd" string is compared here ?*
argv++;
*I don't pass any options ? Do I need to pass some option value ?*
while (argv[0] && argv[0][0]=='-' && argv[0][1]=='-') {
char *opt = argv[0]+2;
*argv++ = (void *)-1;
if (!*opt) {
break;
} else if (!memcmp(opt, "list", 5)) {
ldd_mode = 1;
} else if (!memcmp(opt, "library-path", 12)) {
if (opt[12]=='=') env_path = opt+13;
else if (opt[12]) *argv = 0;
else if (*argv) env_path = *argv++;
} else if (!memcmp(opt, "preload", 7)) {
if (opt[7]=='=') env_preload = opt+8;
else if (opt[7]) *argv = 0;
else if (*argv) env_preload = *argv++;
} else {
argv[0] = 0;
}
}
*I see the argv[0] is set to 0 so I am not able to see my application
loaded ?*

Thanks,
NJ

On Tue, Jan 5, 2016 at 1:23 PM, Rich Felker <dalias@libc.org> wrote:

> On Tue, Jan 05, 2016 at 01:00:37PM -0500, N Jain wrote:
> > > You never answered whether
> > > you were setting up the aux vector right, but if not, that's
> > > definitely going to cause problems.
> >
> > I am not setting any aux vectors. I only pass argv = "app.elf" and argc
> = 1
> > to dynamic linker.
> > What and where I need to set "aux" vectors ? Any pointers will help..
>
> The ELF entry point ABI is that the initial stack pointer points to an
> array of word-sized cells containing:
>
> argc argv[0] argv[1] ... argv[argc-1] argv[argc](=0) anviron[0]
> environ[1] ... 0 auxv[0].key auxv[0].val auxv[1].key auxv[1].val ... 0
>
> The auxv items are key,val pairs where the key is one of the AT_*
> constants from elf.h and the value is either an integer or pointer
> (depending on which key it's for). At the very least you should be
> passing:
>
> AT_PHDR - points to the start of elf program headers
> AT_PHENT - size of each program header
> AT_PHNUM - number of program headers
>
> If you load both the main program and "interpreter" (dynamic linker)
> from the kernel, then these values should be for the main program's
> headers, and in addition you need to pass:
>
> AT_BASE - load address of the "interpreter" (dynamic linker)
> AT_ENTRY - entry point address of the main program, from its header
>
> On the other hand, if you only load the dynamic linker (treating it as
> the main program), then the program header auxv entries should hold
> the right values for the dynamic linker.
>
> Rich
>

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