[Qemu-devel] [PATCH] linux-user: Define AT_RANDOM to support target dynamic linkers that do ASLR

[Cédric VINCENT]

From: Laurent ALFONSI <address@hidden>

The dynamic linker of the GNU C library v2.10+ uses the ELF auxialiary
vector AT_RANDOM as a pointer to a 16-bit random value.  Prior this
patch the value of AT_RANDOM was not defined by the ELF loader of QEMU
so the GNU dynamic linker de-referenced the NULL pointer instead.  As
a consequence any target program linked to the GNU C library v2.10+
crashed due to a SEGFAULT.

Note AT_RANDOM now points to the start of the text segment thus the
16-bit value is not random at all, however it is definitively
readable.  This "dummy" behavior could be improved later.

Signed-off-by: Laurent ALFONSI <address@hidden>
Signed-off-by: Cédric VINCENT <address@hidden>
---

You can easily test this patch with ARMedSlack-13+:

    ftp://ftp.armedslack.org/armedslack/armedslack-devtools/minirootfs/roots/

 linux-user/elfload.c |    9 ++++++++-
 1 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/linux-user/elfload.c b/linux-user/elfload.c
index dcfeb7a..6f67286 100644
--- a/linux-user/elfload.c
+++ b/linux-user/elfload.c
@@ -927,7 +927,7 @@ struct exec
 #define TARGET_ELF_PAGESTART(_v) ((_v) & ~(unsigned 
long)(TARGET_ELF_EXEC_PAGESIZE-1))
 #define TARGET_ELF_PAGEOFFSET(_v) ((_v) & (TARGET_ELF_EXEC_PAGESIZE-1))
 
-#define DLINFO_ITEMS 12
+#define DLINFO_ITEMS 13
 
 static inline void memcpy_fromfs(void * to, const void * from, unsigned long n)
 {
@@ -1271,6 +1271,13 @@ static abi_ulong create_elf_tables(abi_ulong p, int 
argc, int envc,
     NEW_AUX_ENT(AT_EGID, (abi_ulong) getegid());
     NEW_AUX_ENT(AT_HWCAP, (abi_ulong) ELF_HWCAP);
     NEW_AUX_ENT(AT_CLKTCK, (abi_ulong) sysconf(_SC_CLK_TCK));
+
+    /* The dynamic linker of the GNU C library v2.10+ uses the ELF
+     * auxialiary vector AT_RANDOM as a pointer to a 16-bit random
+     * value.  Note the start of the text segment is not random at
+     * all, however it is definitively readeable. */
+    NEW_AUX_ENT(AT_RANDOM, (abi_ulong) info->start_code);
+
     if (k_platform)
         NEW_AUX_ENT(AT_PLATFORM, u_platform);
 #ifdef ARCH_DLINFO
-- 
1.7.5.1