[xen master] char/ns16550: avoid additions to NULL pointer

[patchbot@xxxxxxx]

commit 150b773a484c911e2926f65adb576e1cdf9de7cf
Author:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
AuthorDate: Thu Jul 24 11:42:35 2025 +0200
Commit:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
CommitDate: Thu Jul 24 13:11:47 2025 +0200

    char/ns16550: avoid additions to NULL pointer
    
    Clang UBSAN reports:
    
    UBSAN: Undefined behaviour in drivers/char/ns16550.c:124:49
    applying non-zero offset 0000000000000001 to null pointer
    
    And
    
    UBSAN: Undefined behaviour in drivers/char/ns16550.c:142:49
    applying non-zero offset 0000000000000001 to null pointer
    
    Move calculation of the MMIO address so it's only done if the MMIO is
    mapped by Xen, otherwise the address is not consumed anyway because IO port
    access is used instead.
    
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/drivers/char/ns16550.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/xen/drivers/char/ns16550.c b/xen/drivers/char/ns16550.c
index c1c08b235e..0bbbafb49f 100644
--- a/xen/drivers/char/ns16550.c
+++ b/xen/drivers/char/ns16550.c
@@ -121,11 +121,14 @@ static void cf_check ns16550_delayed_resume(void *data);
 
 static u8 ns_read_reg(const struct ns16550 *uart, unsigned int reg)
 {
-    void __iomem *addr = uart->remapped_io_base + (reg << uart->reg_shift);
+    const volatile void __iomem *addr;
+
 #ifdef CONFIG_HAS_IOPORTS
     if ( uart->remapped_io_base == NULL )
         return inb(uart->io_base + reg);
 #endif
+
+    addr = uart->remapped_io_base + (reg << uart->reg_shift);
     switch ( uart->reg_width )
     {
     case 1:
@@ -139,11 +142,14 @@ static u8 ns_read_reg(const struct ns16550 *uart, 
unsigned int reg)
 
 static void ns_write_reg(const struct ns16550 *uart, unsigned int reg, u8 c)
 {
-    void __iomem *addr = uart->remapped_io_base + (reg << uart->reg_shift);
+    volatile void __iomem *addr;
+
 #ifdef CONFIG_HAS_IOPORTS
     if ( uart->remapped_io_base == NULL )
         return outb(c, uart->io_base + reg);
 #endif
+
+    addr = uart->remapped_io_base + (reg << uart->reg_shift);
     switch ( uart->reg_width )
     {
     case 1:
--
generated by git-patchbot for /home/xen/git/xen.git#master