Re: dlsym(handle) may search in unrelated libraries

[Alexey Izbyshev <izbyshev@ispras.ru>]

On 2/6/19 7:02 PM, Markus Wichmann wrote:
> 
> Thankfully the patch is simple: Explicitly make ldso and vdso have no
> deps. I was tempted to put this into kernel_mapped_dso(), but then I
> remembered that the app is also a kernel mapped dso, and it usually does
> have deps that need processing. At least, in nontrivial cases.
> 
> The attached patch should tide you over.
> 
Thank you for the quick response and the patch, Markus! Your patch fixes 
the exact test case I posted.

Unfortunately, my test case was a simplified example of a general 
problem: dso->deps is assigned only for the main app and for libraries 
opened with dlopen(), but not for their dependencies. Consider the 
following:

$ cat bar.c
int bar = 42;
$ musl-gcc -fPIC -shared bar.c -o libbar.so
$ cat foo.c
extern int bar;
int *foo = &bar;
$ cat baz.c
extern int bazdep;
int *baz = &bazdep;
$ cat bazdep.c
int bazdep = 1;
$ cat main.c
#include <dlfcn.h>
#include <stdio.h>

int main(void) {
   if (!dlopen("libbaz.so", RTLD_NOW|RTLD_LOCAL))
     return 1;
   if (!dlopen("libfoo.so", RTLD_NOW|RTLD_LOCAL))
     return 1;
   void *h = dlopen("libbazdep.so", RTLD_NOW|RTLD_LOCAL);
   printf("%p\n", dlsym(h, "bar"));
}
$ musl-gcc main.c -Wl,-rpath='$ORIGIN' -ldl
$ ./a.out
0x7f66ed371020

Here, "libbazdep.so" assumes the role of "libc.so" from the previous 
test: it's a library with dso->deps == NULL that is loaded before 
"libfoo.so". So, when "libbazdep.so" is dlopen'd, musl considers it to 
be a "first load" and erroneously includes "libbar.so" to the list of 
dependencies of "libbazdep.so".

Alexey