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[Bug 1885332] Re: Error in user-mode calculation of ELF aux vector's AT_
From: |
Thomas Huth |
Subject: |
[Bug 1885332] Re: Error in user-mode calculation of ELF aux vector's AT_PHDR |
Date: |
Wed, 12 May 2021 11:02:15 -0000 |
This is an automated cleanup. This bug report has been moved to QEMU's
new bug tracker on gitlab.com and thus gets marked as 'expired' now.
Please continue with the discussion here:
https://gitlab.com/qemu-project/qemu/-/issues/275
** Changed in: qemu
Status: New => Expired
** Bug watch added: gitlab.com/qemu-project/qemu/-/issues #275
https://gitlab.com/qemu-project/qemu/-/issues/275
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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:
Expired
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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