[xen staging] xen/console: Fix truncation of panic() messages

[patchbot@xxxxxxx]

commit 81f8b1dd9407e4a3d9dc058b7fbbc591168649ad
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Wed Jan 22 12:13:24 2025 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Feb 18 14:15:58 2025 +0000

    xen/console: Fix truncation of panic() messages
    
    The panic() function uses a static buffer to format its arguments into, 
simply
    to emit the result via printk("%s", buf).  This buffer is not large enough 
for
    some existing users in Xen.  e.g.:
    
      (XEN) ****************************************
      (XEN) Panic on CPU 0:
      (XEN) Invalid device tree blob at physical address 0x46a00000.
      (XEN) The DTB must be 8-byte aligned and must not exceed 2 MB in size.
      (XEN)
      (XEN) Plea****************************************
    
    The remainder of this particular message is 'e check your bootloader.', but
    has been inherited by RISC-V from ARM.
    
    It is also pointless double buffering.  Implement vprintk() beside printk(),
    and use it directly rather than rendering into a local buffer, removing it 
as
    one source of message limitation.
    
    This marginally simplifies panic(), and drops a global used-once buffer.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
---
 xen/drivers/char/console.c | 21 +++++++++++++--------
 xen/include/xen/lib.h      |  2 ++
 2 files changed, 15 insertions(+), 8 deletions(-)

diff --git a/xen/drivers/char/console.c b/xen/drivers/char/console.c
index 7da8c5296f..2926c97df9 100644
--- a/xen/drivers/char/console.c
+++ b/xen/drivers/char/console.c
@@ -962,11 +962,17 @@ static void vprintk_common(const char *prefix, const char 
*fmt, va_list args)
     local_irq_restore(flags);
 }
 
+void vprintk(const char *fmt, va_list args)
+{
+    vprintk_common("(XEN) ", fmt, args);
+}
+
 void printk(const char *fmt, ...)
 {
     va_list args;
+
     va_start(args, fmt);
-    vprintk_common("(XEN) ", fmt, args);
+    vprintk(fmt, args);
     va_end(args);
 }
 
@@ -1268,23 +1274,22 @@ void panic(const char *fmt, ...)
     va_list args;
     unsigned long flags;
     static DEFINE_SPINLOCK(lock);
-    static char buf[128];
 
     spin_debug_disable();
     spinlock_profile_printall('\0');
     debugtrace_dump();
 
-    /* Protects buf[] and ensure multi-line message prints atomically. */
+    /* Ensure multi-line message prints atomically. */
     spin_lock_irqsave(&lock, flags);
 
-    va_start(args, fmt);
-    (void)vsnprintf(buf, sizeof(buf), fmt, args);
-    va_end(args);
-
     console_start_sync();
     printk("\n****************************************\n");
     printk("Panic on CPU %d:\n", smp_processor_id());
-    printk("%s", buf);
+
+    va_start(args, fmt);
+    vprintk(fmt, args);
+    va_end(args);
+
     printk("****************************************\n\n");
     if ( opt_noreboot )
         printk("Manual reset required ('noreboot' specified)\n");
diff --git a/xen/include/xen/lib.h b/xen/include/xen/lib.h
index 81b722ea3e..130f96791f 100644
--- a/xen/include/xen/lib.h
+++ b/xen/include/xen/lib.h
@@ -60,6 +60,8 @@ debugtrace_printk(const char *fmt, ...) {}
 
 extern void printk(const char *fmt, ...)
     __attribute__ ((format (printf, 1, 2), cold));
+void vprintk(const char *fmt, va_list args)
+    __attribute__ ((format (printf, 1, 0), cold));
 
 #define printk_once(fmt, args...)               \
 ({                                              \
--
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