[PATCH v2 1/5] x86/wait: prevent duplicated assembly labels

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

When enabling UBSAN with clang, the following error is triggered during the
build:

common/wait.c:154:9: error: symbol '.L_wq_resume' is already defined
  154 |         "push %%rbx; push %%rbp; push %%r12;"
      |         ^
<inline asm>:1:121: note: instantiated into assembly here
    1 |         push %rbx; push %rbp; push %r12;push %r13; push %r14; push 
%r15;sub %esp,%ecx;cmp $4096, %ecx;ja .L_skip;mov %rsp,%rsi;.L_wq_resume: rep 
movsb;mov %rsp,%rsi;.L_skip:pop %r15; pop %r14; pop %r13;pop %r12; pop %rbp; 
pop %rbx
      |                                                                         
                                                       ^
common/wait.c:154:9: error: symbol '.L_skip' is already defined
  154 |         "push %%rbx; push %%rbp; push %%r12;"
      |         ^
<inline asm>:1:159: note: instantiated into assembly here
    1 |         push %rbx; push %rbp; push %r12;push %r13; push %r14; push 
%r15;sub %esp,%ecx;cmp $4096, %ecx;ja .L_skip;mov %rsp,%rsi;.L_wq_resume: rep 
movsb;mov %rsp,%rsi;.L_skip:pop %r15; pop %r14; pop %r13;pop %r12; pop %rbp; 
pop %rbx
      |                                                                         
                                                                                
             ^
2 errors generated.

The inline assembly block in __prepare_to_wait() is duplicated, thus
leading to multiple definitions of the otherwise unique labels inside the
assembly block.  GCC extended-asm documentation notes the possibility of
duplicating asm blocks:

> Under certain circumstances, GCC may duplicate (or remove duplicates of)
> your assembly code when optimizing. This can lead to unexpected duplicate
> symbol errors during compilation if your asm code defines symbols or
> labels. Using ‘%=’ (see AssemblerTemplate) may help resolve this problem.

Workaround the issue by latching esp to a local variable, this prevents
clang duplicating the inline asm blocks.

Suggested-by: Jan Beulich <jbeulich@xxxxxxxx>
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v1:
 - Use approach suggested by Jan.
---
 xen/common/wait.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/xen/common/wait.c b/xen/common/wait.c
index cb6f5ff3c20a..9a11dccb5de5 100644
--- a/xen/common/wait.c
+++ b/xen/common/wait.c
@@ -124,6 +124,11 @@ static void __prepare_to_wait(struct waitqueue_vcpu *wqv)
     struct cpu_info *cpu_info = get_cpu_info();
     struct vcpu *curr = current;
     unsigned long dummy;
+    /*
+     * Latch esp to a local variable to prevent clang from duplicating the
+     * inline assembly block when UBSAN is enabled.
+     */
+    void *esp = NULL;
 
     ASSERT(wqv->esp == NULL);
 
@@ -166,12 +171,12 @@ static void __prepare_to_wait(struct waitqueue_vcpu *wqv)
         ".L_skip:"
         "pop %%r15; pop %%r14; pop %%r13;"
         "pop %%r12; pop %%rbp; pop %%rbx"
-        : "=&S" (wqv->esp), "=&c" (dummy), "=&D" (dummy)
+        : "=&S" (esp), "=&c" (dummy), "=&D" (dummy)
         : "0" (0), "1" (cpu_info), "2" (wqv->stack),
           [sz] "i" (PAGE_SIZE)
         : "memory", "rax", "rdx", "r8", "r9", "r10", "r11" );
 
-    if ( unlikely(wqv->esp == NULL) )
+    if ( unlikely(esp == NULL) )
     {
         gdprintk(XENLOG_ERR, "Stack too large in %s\n", __func__);
         domain_crash(curr->domain);
@@ -179,6 +184,7 @@ static void __prepare_to_wait(struct waitqueue_vcpu *wqv)
         for ( ; ; )
             do_softirq();
     }
+    wqv->esp = esp;
 }
 
 static void __finish_wait(struct waitqueue_vcpu *wqv)
-- 
2.48.1