Re: RFC: Add DT_FLAGS_2 and DF_2_GNU_IFUNC

[Florian Weimer <fweimer at redhat dot com>]

On 05/25/2018 11:58 PM, H.J. Lu wrote:
> On Fri, May 25, 2018 at 2:26 PM, Florian Weimer <fweimer@redhat.com> wrote:
> > On 05/25/2018 11:18 PM, H.J. Lu wrote:
> >
> > > > Still I don't see what this buys you if the LD_PRELOAD object has IFUNCs,
> > > > too.
> > >
> > >
> > > [hjl@gnu-cfl-1 ifunc-2]$ cat bar.c
> > > void
> > > bar (char *dst, const char *src, unsigned int size)
> > > {
> > >     __builtin_memmove (dst, src, size);
> > > }
> > > [hjl@gnu-cfl-1 ifunc-2]$ cat main.c
> > > extern void bar (char *, const char *, unsigned int);
> > >
> > > int
> > > main ()
> > > {
> > >     char dst[50];
> > >     const char src[] =
> > >       {
> > >         "This is a test"
> > >       };
> > >     bar (dst, src, sizeof (src));
> > >     if (__builtin_memcmp (dst, src, sizeof (src)) != 0)
> > >       __builtin_abort ();
> > >     return 0;
> > > }
> > > [hjl@gnu-cfl-1 ifunc-2]$ make
> > > gcc -O2 -g   -c -o main.o main.c
> > > gcc -O2 -g -fPIC   -c -o bar.o bar.c
> > > gcc -shared -z now -o libbar.so bar.o
> > > gcc -z now -o x main.o libbar.so -Wl,-R,.
> > > ./x
> > > [hjl@gnu-cfl-1 ifunc-2]$ LD_PRELOAD=./libmemmove.so ./x
> > > ./x: Relink `./libbar.so' with `./libmemmove.so' for IFUNC symbol
> > > `memmove'
> > > [hjl@gnu-cfl-1 ifunc-2]$
> >
> >
> > This doesn't reproduce for me.  The commands above do not produce
> > libmemmove.so.  “LD_PRELOAD=./libbar.so ./x” does not crash here (with a
> > glibc 2.26-derived library that hasn't got my IFUNC scheduler).

(some commands are missing from the reproduction steps)

> https://sourceware.org/bugzilla/show_bug.cgi?id=23240

This is just the old known issue related to symbol interposition and 
IFUNC resolvers, where even complete DT_NEEDED information is not 
sufficient to perform the relocations in the correct order.

A single bit will not solve this.  In your reproducer, add an IFUNC 
resolver to libbar.so, and I'm sure the bug will resurface again.

Thanks,
Florian