Wrap all functions accessing /etc/passwd, /etc/group and /etc/shadow for glibc >= 2.34

[josch]

closes: #97

Starting with glibc 2.32 the compat nss module for getpwnam calls
__nss_files_fopen (which is a GLIBC_PRIVATE symbol provided by glibc)
instead of fopen (see 299210c1fa67e2dfb564475986fce11cd33db9ad). This
leads to getpwnam calls accessing /etc/passwd from outside the chroot
and as a result programs like adduser do not work correctly anymore
under fakechroot.

Starting with glibc 2.34 the __nss_files_fopen was moved from nss to
libc.so and thus wrapping it with LD_PRELOAD has no affect anymore
(see 6212bb67f4695962748a5981e1b9fea105af74f6).

So now we also wrap all the functions accessing /etc/passwd, /etc/group
and /etc/shadow. This solution will ignore NIS, LDAP or other local files
as potentially configured in /etc/nsswitch.conf.

[josch]

Starting with glibc 2.34, getpwnam and friends fail again. Probably the modules changed again in a subtle way which makes wrapping __nss_files_fopen not effective anymore. But so far I fail to figure out the root of the problem because a backtrace of gdb at the point when /etc/passwd is opened doesn't show a different call stack compared to glibc 2.33:

#0  __GI___open64_nocancel (file=0x7ffff7f73dc5 "/etc/passwd", oflag=524288)
    at ../sysdeps/unix/sysv/linux/open64_nocancel.c:45
#1  0x00007ffff7e3f265 in __GI__IO_file_open (fp=fp@entry=0x555555559790, 
    filename=filename@entry=0x7ffff7f73dc5 "/etc/passwd", posix_mode=<optimized out>, 
    prot=prot@entry=438, read_write=8, is32not64=<optimized out>) at ./libio/fileops.c:186
#2  0x00007ffff7e3f38b in _IO_new_file_fopen (fp=fp@entry=0x555555559790, 
    filename=filename@entry=0x7ffff7f73dc5 "/etc/passwd", mode=<optimized out>, 
    mode@entry=0x7ffff7f6fb14 "rce", is32not64=is32not64@entry=1) at ./libio/fileops.c:281
#3  0x00007ffff7e33a09 in __fopen_internal (filename=filename@entry=0x7ffff7f73dc5 "/etc/passwd", 
    mode=mode@entry=0x7ffff7f6fb14 "rce", is32=is32@entry=1) at ./libio/iofopen.c:75
#4  0x00007ffff7e33a8a in _IO_new_fopen (filename=filename@entry=0x7ffff7f73dc5 "/etc/passwd", 
    mode=mode@entry=0x7ffff7f6fb14 "rce") at ./libio/iofopen.c:86
#5  0x00007ffff7ef305e in __GI___nss_files_fopen (path=path@entry=0x7ffff7f73dc5 "/etc/passwd")
    at ./nss/nss_files_fopen.c:27
#6  0x00007ffff7ef8229 in internal_setent (stream=<synthetic pointer>) at nss_files/files-XXX.c:76
#7  __GI__nss_files_getpwnam_r (name=0x7fffffffe85b "user", result=0x7ffff7fb8a00 <resbuf>, 
    buffer=0x5555555592a0 "", buflen=1024, errnop=0x7ffff7dbc6c0) at nss_files/files-pwd.c:33
#8  0x00007ffff7e93bf8 in __getpwnam_r (name=name@entry=0x7fffffffe85b "user", 
    resbuf=resbuf@entry=0x7ffff7fb8a00 <resbuf>, buffer=<optimized out>, buflen=buflen@entry=1024, 
    result=result@entry=0x7fffffffe4b0) at ../nss/getXXbyYY_r.c:274
#9  0x00007ffff7e9368c in getpwnam (name=0x7fffffffe85b "user") at ../nss/getXXbyYY.c:135
#10 0x00005555555550f4 in main (argc=<optimized out>, argv=<optimized out>)
    at test-nss_files_fopen.c:16

Here is a small test program to illustrate the problem:

#include <stdlib.h>
#include <stdio.h>
#include <pwd.h>
#include <errno.h>
#include <stdint.h>
#include <unistd.h>

int main (int argc, char *argv[]) {
	struct passwd *pwd;

	if (argc != 2) {
		fprintf(stderr, "Usage: %s username\n", argv[0]);
		exit(EXIT_FAILURE);
	}

	pwd = getpwnam(argv[1]);
	if (pwd == NULL) {
		if (errno == 0) {
			printf("Not found\n");
		} else {
			perror("getpwnam");
		}
		exit(EXIT_FAILURE);
	}

	printf("%jd\n", (intmax_t)(pwd->pw_uid));
	exit(EXIT_SUCCESS);
}

Running this inside a fakechroot with FAKECHROOT_DEBUG=1 shows that fakechroot doesn't wrap anything this test program should open (like /etc/passwd). Instead, the uid that this test program will print, will come from the host system and not from inside the chroot.

@sergiomb2: are you seeing the same problems in Fedora?

EDIT:
This glibc commit broke this: https://sourceware.org/git/?p=glibc.git;a=commit;h=6212bb67f4695962748a5981e1b9fea105af74f6
Debian bug: https://bugs.debian.org/1017590

[josch]

Since __nss_files_fopen is part of libc6.so, wrapping it with LD_PRELOAD doesn't have an effect anymore. So instead we wrap all functions accessing /etc/passwd, /etc/group and /etc/shadow.

[sergiomb2]

Hi, I don't know , I maintain fakechroot because could be used by pbuilder and debootstrap , but digging today I also don't know if it is required .