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problem on /proc/self/exe and /proc/num/exe

In my project, I encountered a strange problem. As you know readlink(2) can be used to get the real running program by /proc/self/exe. For example:

on RHELU4 and RH9
test.c:
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>

int main()
{
char elf_file[65536];
pid_t pid;
char string[100];

pid = getpid();
printf("pid is %ld\n", pid);

sprintf(string, "/proc/%ld/exe", pid);
printf("string is %s\n", string);
sleep(20);
if (readlink(string, elf_file, 65536) == -1) {
perror("readlink");
return -1;
}
printf("real file is %s\n", elf_file);
}
compiled by "gcc test.c -o test -static". On ia64, x86_64 and i386, it's all OK. But if I run test as a ld, compiled with "gcc -Wl,--dynamic-linker test hello.c -o hello". It will fail on i386 and x86_64, but OK on ia64.
[root@hpc-rm ld]# ./hello
pid is 22226
string is /proc/22226/exe
readlink: No such file or directory
[root@hpc-rm ld]# ./test
pid is 22239
string is /proc/22239/exe
real file is /home/baiwd/ld/test
And on i386 and x86_64, when I "ll /proc/pid[num]/", there is /proc/pid/exe, but when I "ll /proc/pid[num]/exe", it reports "No such file or directory". Could you please tell me why? Thank you very much

Try using code tags around you source in the posts, makes it much easier to read.

readlink() does not place a null terminator on the end, you have to do that yourself using the return code as the index, after range checking it of course. See the man 2 readlink.

Thank you. But I am afraid that maybe it's not caused by readlink(2). In fact the "/proc/pidnumber/exe" does not exist, because when using command "ll /proc/pidnumber/exe", it reports "No such file or directory" too. So readlink failed.

Your program works for me after I fixed the readlink() problem.
I don't know what error you are seeing.

Code:

#include <stdio.h>

#include <unistd.h>

#include <sys/types.h>



int main() {

    char elf_file[65536];

    pid_t pid;

    char string[100];

    int res;





    pid=getpid();

    printf("pid is %d\n", pid);

    snprintf(string,sizeof(string),"/proc/%d/exe",pid);

    printf("string is %s\n", string);

    sleep(20);

    res=readlink(string,elf_file,sizeof(elf_file));

    if (res<=0 || res>=sizeof(elf_file)) {

        perror("readlink");

        return 1;

    }

    elf_file[res]='\0';

    printf("real file is %s\n",elf_file);

    return 0;

}

if I run the program test directly, it's all right, every thing is OK. But I want to use test as a faked linker and I want to figure out which program is running in this faked ld. But the /proc/<pid>/exe does not exist.

Quote:

[baiwd@hpc-rm ld]$ gcc -Wl,--dynamic-linker test hello.c -o hello
[baiwd@hpc-rm ld]$ cat hello.c
#include <stdio.h>
int main()
{
printf("hello\n");
return 0;
}
[baiwd@hpc-rm ld]$ ./hello
pid is 19738
string is /proc/19738/exe
readlink: No such file or directory
[root@hpc-rm ld]# ll /proc/19738/exe
ls: cannot read symbolic link /proc/19738/exe: No such file or directory
lrwxrwxrwx 1 baiwd baiwd 0 Aug 31 11:01 /proc/19738/exe

Hmm, sorry, the ld man page says this about --dynamic-linker

Code:

The default dynamic linker is normally correct;

don't use this unless you know what you are doing.

Since I never heard of this option, I probably don't know what I'm doing.
Maybe there's someone else that can assist you.

Thank you for your sparing time. Now another nice person give me the answer:

Quote:

The problem is that your "test" ld is linked to load at the same
address as hello:

$ objdump -p ./hello | grep LOAD | head -1
LOAD off 0x00000000 vaddr 0x08048000 paddr 0x08048000 align 2**12
$ objdump -p ./test | grep LOAD | head -1
LOAD off 0x00000000 vaddr 0x08048000 paddr 0x08048000 align 2**12

You have to get your "test ld" out of the way by linking it at a
different address (using a linker script), or get your "hello"
executable out of the way by building it as a position-independent:

$ gcc -g junk.c -o test -static &&
gcc junk.c -Wl,--dynamic-linker=./test &&
./a.out
pid is 16060
string is /proc/16060/exe
readlink: No such file or directory

$ gcc -fPIC -pie junk.c -Wl,--dynamic-linker=./test &&
./a.out
pid is 16066
string is /proc/16066/exe
real file is /home/paul/a.out