[Xen-devel] [PATCH v4 03/16] arm: poison initmem when it is freed.

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

The current byte sequence is '0xcc' which makes sense on x86,
but on ARM it is:

cccccccc        stclgt  12, cr12, [ip], {204}   ; 0xcc

Picking something more ARM applicable such as:

efefefef        svc     0x00efefef

Creates a nice crash if one executes that code:
(XEN) CPU1: Unexpected Trap: Supervisor Call

But unfortunatly that may not be a good choice either as in the future
we may want to implement support for it.

Julien suggested that we use a 4-byte insn instruction instead
of trying to work with one byte.

As such on ARM 32 we use the udf instruction (see A8.8.247
in ARM DDI 0406C.c) and on ARM 64 use the AARCH64_BREAK_FAULT
instruction (aka brk instruction).

We don't have to worry about Thumb code so this instruction
is a safe to execute.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
---
Cc: Julien Grall <julien.grall@xxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>

v3: New submission
v4: Instead of using 0xef, use specific insn for architectures.
---
 xen/arch/arm/mm.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
index 07e2037..438bed7 100644
--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -994,8 +994,23 @@ void free_init_memory(void)
 {
     paddr_t pa = virt_to_maddr(__init_begin);
     unsigned long len = __init_end - __init_begin;
+    uint32_t insn;
+    unsigned int i, nr = len / sizeof(insn);
+
     set_pte_flags_on_range(__init_begin, len, mg_rw);
-    memset(__init_begin, 0xcc, len);
+#ifdef CONFIG_ARM_32
+    /* udf instruction i.e (see A8.8.247 in ARM DDI 0406C.c) */
+    insn = 0xe7f000f0;
+#else
+    insn = AARCH64_BREAK_FAULT;
+#endif
+    for ( i = 0; i < nr; i++ )
+        *(__init_begin + i) = insn;
+
+    nr = len % sizeof(insn);
+    if ( nr )
+        memset(__init_begin + len - nr, 0xcc, nr);
+
     set_pte_flags_on_range(__init_begin, len, mg_clear);
     init_domheap_pages(pa, pa + len);
     printk("Freed %ldkB init memory.\n", (long)(__init_end-__init_begin)>>10);
-- 
2.5.5


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