[Bug 1885332] Re: Error in user-mode calculation of ELF aux vector's AT_PHDR

[Thomas Huth <1885332@bugs.launchpad.net>]

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** Changed in: qemu
       Status: New => Incomplete

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https://bugs.launchpad.net/bugs/1885332

Title:
  Error in user-mode calculation of ELF aux vector's AT_PHDR

Status in QEMU:
  Incomplete

Bug description:
  
  I have an (admittedly strange) statically-linked ELF binary for Linux that runs just fine on top of the Linux kernel in QEMU full-system emulation, but crashes before main in user-mode emulation. Specifically, it crashes when initializing thread-local storage in glibc's _dl_aux_init, because it reads out a strange value from the AT_PHDR entry of the ELF aux vector.

  The binary has these program headers:

    Program Headers:
      Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
      EXIDX          0x065874 0x00075874 0x00075874 0x00570 0x00570 R   0x4
      PHDR           0x0a3000 0x00900000 0x00900000 0x00160 0x00160 R   0x1000
      LOAD           0x0a3000 0x00900000 0x00900000 0x00160 0x00160 R   0x1000
      LOAD           0x000000 0x00010000 0x00010000 0x65de8 0x65de8 R E 0x10000
      LOAD           0x066b7c 0x00086b7c 0x00086b7c 0x02384 0x02384 RW  0x10000
      NOTE           0x000114 0x00010114 0x00010114 0x00044 0x00044 R   0x4
      TLS            0x066b7c 0x00086b7c 0x00086b7c 0x00010 0x00030 R   0x4
      GNU_STACK      0x000000 0x00000000 0x00000000 0x00000 0x00000 RW  0x8
      GNU_RELRO      0x066b7c 0x00086b7c 0x00086b7c 0x00484 0x00484 R   0x1
      LOAD           0x07e000 0x00089000 0x00089000 0x03f44 0x03f44 R E 0x1000
      LOAD           0x098000 0x00030000 0x00030000 0x01000 0x01000 RW  0x1000

  If I build the Linux kernel with the following patch to the very end
  of create_elf_tables in fs/binfmt_elf.c

    /* Put the elf_info on the stack in the right place.  */
    elf_addr_t *my_auxv = (elf_addr_t *) mm->saved_auxv;
    int i;
    for (i = 0; i < 15; i++) {
      printk("0x%x = 0x%x", my_auxv[2*i], my_auxv[(2*i)+ 1]);
    }
    if (copy_to_user(sp, mm->saved_auxv, ei_index * sizeof(elf_addr_t)))
        return -EFAULT;
    return 0;

  and run it like this:

    qemu-system-arm \
      -M versatilepb \
      -nographic \
      -dtb ./dts/versatile-pb.dtb \
      -kernel zImage \
      -M versatilepb \
      -m 128M \
      -append "earlyprintk=vga,keep" \
      -initrd initramfs

  after I've built the kernel initramfs like this (where "init" is the
  binary in question):

    make ARCH=arm versatile_defconfig
    make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- all -j10
    cp "$1" arch/arm/boot/init
    cd arch/arm/boot
    echo init | cpio -o --format=newc > initramfs

  then I get the following output. This is the kernel's view of the aux
  vector for this binary:

    0x10 = 0x1d7
    0x6 = 0x1000
    0x11 = 0x64
    0x3 = 0x900000
    0x4 = 0x20
    0x5 = 0xb
    0x7 = 0x0
    0x8 = 0x0
    0x9 = 0x101b8
    0xb = 0x0
    0xc = 0x0
    0xd = 0x0
    0xe = 0x0
    0x17 = 0x0
    0x19 = 0xbec62fb5

  However, if I run "qemu-arm -g 12345 binary" and use GDB to peek at
  the aux vector at the beginning of __libc_start_init (for example,
  using this Python GDB API script: https://gist.github.com/langston-
  barrett/5573d64ae0c9953e2fa0fe26847a5e1e), then I see the following
  values:

    AT_PHDR = 0xae000
    AT_PHENT = 0x20
    AT_PHNUM = 0xb
    AT_PAGESZ = 0x1000
    AT_BASE = 0x0
    AT_FLAGS = 0x0
    AT_ENTRY = 0x10230
    AT_UID = 0x3e9
    AT_EUID = 0x3e9
    AT_GID = 0x3e9
    AT_EGID = 0x3e9
    AT_HWCAP = 0x1fb8d7
    AT_CLKTCK = 0x64
    AT_RANDOM = -0x103c0
    AT_HWCAP2 = 0x1f
    AT_NULL = 0x0

  The crucial difference is in AT_PHDR (0x3), which is indeed the
  virtual address of the PHDR segment when the kernel calculates it, but
  is not when QEMU calculates it.

  qemu-arm --version
  qemu-arm version 2.11.1(Debian 1:2.11+dfsg-1ubuntu7.26)

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