readlink qemu linux-user handling

log

$ sudo arch-chroot ./ /qemu-x86_64-static /main
==> WARNING: ./ is not a mountpoint. This may have undesirable side effects.
readlink(/proc/self/exe) => /main (len: 5)
readlink(/proc/self) => 27679 (len: 5)
readlink(/proc/1/exe) => /main (len: 5)
realpath(/proc/self/exe) => /qemu-x86_64-static
AT_EXECFN = /main

$ sudo arch-chroot ./ /qemu-x86_64-static -strace /main
==> WARNING: ./ is not a mountpoint. This may have undesirable side effects.
1 arch_prctl(12289,274886296688,4320976,2561,3,2048) = -1 errno=22 (Invalid argument)
1 brk(NULL) = 0x00000000004aa888
1 brk(0x00000000004ab588) = 0x00000000004ab588
1 arch_prctl(4098,4893760,24,2561,2,4892808) = 0
1 set_tid_address(0x4aaf10) = 1
1 set_robust_list(0x4aaf20,24) = -1 errno=38 (Function not implemented)
1 Unknown syscall 334
1 prlimit64(0,RLIMIT_STACK,NULL,0x0000004000800390) = 0 ({rlim_cur=8388608,rlim_max=-1})
1 readlinkat(-100,"/proc/self/exe",0x00000040007ff310,4096) = 5
1 getrandom(0x4a9d70,8,1) = 8
1 brk(NULL) = 0x00000000004ab588
1 brk(0x00000000004cc588) = 0x00000000004cc588
1 brk(0x00000000004cd000) = 0x00000000004cd000
1 mprotect(0x000000000049f000,16384,PROT_READ) = 0
1 readlink("/proc/self/exe",0x00000040007fd2b0,4096) = 5
1 newfstatat(1,"",0x00000040007fcf00,0x1000) = 0
1 write(1,0x4aba70,43)readlink(/proc/self/exe) => /main (len: 5)
 = 43
1 readlink("/proc/self",0x00000040007fd2b0,4096) = 5
1 write(1,0x4aba70,39)readlink(/proc/self) => 27806 (len: 5)
 = 39
1 getpid() = 1
1 readlink("/proc/1/exe",0x00000040007fd2b0,4096) = 5
1 write(1,0x4aba70,40)readlink(/proc/1/exe) => /main (len: 5)
 = 40
1 readlink("/proc",0x00000040007fde80,1023) = -1 errno=22 (Invalid argument)
1 readlink("/proc/self",0x00000040007fde80,1023) = 5
1 readlink("/proc/27806",0x00000040007fde80,1023) = -1 errno=22 (Invalid argument)
1 readlink("/proc/27806/exe",0x00000040007fde80,1023) = 19
1 readlink("/qemu-x86_64-static",0x00000040007fde80,1023) = -1 errno=22 (Invalid argument)
1 write(1,0x4aba70,48)realpath(/proc/self/exe) => /qemu-x86_64-static
 = 48
1 write(1,0x4aba70,18)AT_EXECFN = /main
 = 18
1 exit_group(0)

readlink-qemu.c

#include <stdio.h>
#include <limits.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <sys/auxv.h>

void do_readlink(const char * path)
{
    char buf[PATH_MAX];
    ssize_t len = readlink(path, buf, PATH_MAX);
    if (len > 0)
        printf("readlink(%s) => %.*s (len: %zu)\n", path, (int)len, buf, len);
    else
        printf("readlink(%s) = -%s\n", path, strerror(errno));
}

int main()
{
    do_readlink("/proc/self/exe");
    do_readlink("/proc/self");
    {
        char path[PATH_MAX];
        snprintf(path, PATH_MAX, "/proc/%d/exe", getpid());
        do_readlink(path);
    }

    {
        char exe[PATH_MAX];
        realpath("/proc/self/exe", exe);
        printf("realpath(/proc/self/exe) => %s\n", exe);
    }

    const char * execfn = (char *)getauxval(AT_EXECFN);
    printf("AT_EXECFN = %s\n", execfn);
}