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Re: [PATCH for-4.20? 1/4] ARM32/traps: Fix do_trap_undefined_instruction()'s detection of kernel text
Hi Andrew,
On 08/02/2025 00:02, Andrew Cooper wrote:
While fixing some common/arch boundaries for UBSAN support on other
architectures, the following debugging patch:
diff --git a/xen/arch/arm/setup.c b/xen/arch/arm/setup.c
index c1f2d1b89d43..58d1d048d339 100644
--- a/xen/arch/arm/setup.c
+++ b/xen/arch/arm/setup.c
@@ -504,6 +504,8 @@ void asmlinkage __init start_xen(unsigned long fdt_paddr)
system_state = SYS_STATE_active;
+ dump_execution_state();
+
for_each_domain( d )
domain_unpause_by_systemcontroller(d);
fails with:
(XEN) *** Serial input to DOM0 (type 'CTRL-a' three times to switch input)
(XEN) CPU0: Unexpected Trap: Undefined Instruction
(XEN) ----[ Xen-4.20-rc arm32 debug=n Not tainted ]----
(XEN) CPU: 0
<snip>
(XEN)
(XEN) ****************************************
(XEN) Panic on CPU 0:
(XEN) CPU0: Unexpected Trap: Undefined Instruction
(XEN) ****************************************
This is because the condition for init text is wrong. While there's nothing
interesting from that point onwards in start_xen(), it's also wrong for any
livepatch which brings in an adjusted BUG_FRAME().
Use is_active_kernel_text() which is the correct test for this purpose, and is
aware of init and livepatch regions too.
Commit c8d4b6304a5e ("xen/arm: add support for run_in_exception_handler()"),
made run_in_exception_handler() work, but didn't complete the TODO left in
commit 3e802c6ca1fb ("xen/arm: Correctly support WARN_ON"). Do so, to make
ARM consistent with other architectures.
This was done on purpose. If you look at the current implementation of
run_in_exception_handler(), it will clobber some registers.
With your patch #2, the function should only clobber one. It is a bit
better, but it still not great. So I think we need to stick with WARN()
on Arm (+ maybe a comment explaning why it is implemented differently).
Cheers,
--
Julien Grall
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