Regression: NULL l_addr in link_map structure for vdso on 5.5.0

Start with the minimal vdsoaddr application and reproduce it under rr, comparing the link_map l_addr value with and without rr. Read elf/setup-vdso.h, especially setup_vdso(), and inspect the librrpage.so PT_LOAD entries and commit 4be0255d. Done means the vdso link_map l_addr remains valid under rr without regressing normal execution.

I've discovered a regression from 5.4.0 to 5.5.0. Applications that use dlopen() to obtain a dynamic linker struct link_map pointer corresponding to the vdso and observe the l_addr field will find it to be NULL. Here's a minimal application that crashes on 5.5.0 (link with -ldl):

#include <assert.h>
#include <link.h>
#include <string.h>

int main(void) {
	const struct link_map *l = dlopen(NULL, RTLD_LAZY);
	while(l && strcmp(l->l_name, "linux-vdso.so.1"))
		l = l->l_next;
	assert(l->l_addr);

	return 0;
}

It looks like the NULL originates in ld-linux.so's setup_vdso() function. When running without rr, all of the PT_LOAD entries in the program header have a p_vaddr of 0, so line 52 has no effect and line 64 is equivalent to l->l_addr = l->l_map_start. Under rr, the first PT_LOAD entry has a p_vaddr that matches l->l_map_start, so line 64 instead resets l->l_addr to NULL.

It looks like for setup_vdso() to work correctly on librrpage.so, all of its PT_LOAD entries would have to have a p_vaddr of 0, which is infeasible. And since setup_vdso() is an inline function, it cannot be interposed. Perhaps rr can just manually set the l_addr field after the dynamic linker is finished?

I'm using glibc 2.33 from Debian:

$ apt policy libc6
libc6:
  Installed: 2.33-1

It looks like this was introduced in 4be0255d, which proxies the vdso. Here's my bisection script:

#!/bin/sh

mydir="`dirname "$0"`"
set -ev
[ -e vdsoaddr ] || c99 -o vdsoaddr "$mydir/vdsoaddr.c" -ldl
[ -e vdsoaddr-0 ] && rm -rf vdsoaddr-0
git cherry HEAD dbc94c6f | grep ^+ >/dev/null && git cherry-pick -n dbc94c6f
./configure
make -j8
git reset --hard
_RR_TRACE_DIR="." bin/rr ./vdsoaddr || false