[xen master] x86/PV: replace assertions in '0' debug key stack dumping

[patchbot@xxxxxxx]

commit 525eac931794434593c39a1d1cd739ad8b326e27
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Oct 19 10:07:42 2021 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Oct 19 10:07:42 2021 +0200

    x86/PV: replace assertions in '0' debug key stack dumping
    
    While it was me to add them, I'm afraid I don't see justification for
    the assertions: A vCPU may very well have got preempted while in user
    mode. Limit compat guest user mode stack dumps to the containing page
    (like is done when using do_page_walk()), and suppress user mode stack
    dumping altogether for 64-bit domains.
    
    Fixes: cc0de53a903c ("x86: improve output resulting from sending '0' over 
serial")
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/traps.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index 4f3a7131d6..a1c2adb7ad 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -254,7 +254,6 @@ static void compat_show_guest_stack(struct vcpu *v,
         struct vcpu *vcpu;
         unsigned long mfn;
 
-        ASSERT(guest_kernel_mode(v, regs));
         mfn = read_cr3() >> PAGE_SHIFT;
         for_each_vcpu( v->domain, vcpu )
             if ( pagetable_get_pfn(vcpu->arch.guest_table) == mfn )
@@ -269,6 +268,8 @@ static void compat_show_guest_stack(struct vcpu *v,
             }
             mask = PAGE_SIZE;
         }
+        else if ( !guest_kernel_mode(v, regs) )
+            mask = PAGE_SIZE;
     }
 
     for ( i = 0; i < debug_stack_lines * 8; i++ )
@@ -328,7 +329,12 @@ static void show_guest_stack(struct vcpu *v, const struct 
cpu_user_regs *regs)
     {
         struct vcpu *vcpu;
 
-        ASSERT(guest_kernel_mode(v, regs));
+        if ( !guest_kernel_mode(v, regs) )
+        {
+            printk("User mode stack\n");
+            return;
+        }
+
         vcpu = maddr_get_owner(read_cr3()) == v->domain ? v : NULL;
         if ( !vcpu )
         {
--
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