[Qemu-devel] [PATCH v2] 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 word with 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
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>
---
 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..111128c 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
+     * auxiliary vector AT_RANDOM as a pointer to a word with random
+     * value.  Note the start of the text segment is not random at
+     * all, however it is definitively readable.  */
+    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