[xen stable-4.19] xen/console: Fix truncation of panic() messages

[patchbot@xxxxxxx]

commit 330ecfc7c1bea7699d3783c8d733cab53e933d39
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Thu Mar 20 13:13:44 2025 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Mar 20 13:13:44 2025 +0100

    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>
    master commit: 81f8b1dd9407e4a3d9dc058b7fbbc591168649ad
    master date: 2025-02-18 14:15:58 +0000
---
 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 3a3a97bcbe..a18cf7dfa0 100644
--- a/xen/drivers/char/console.c
+++ b/xen/drivers/char/console.c
@@ -961,11 +961,17 @@ 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);
 }
 
@@ -1267,23 +1273,22 @@ 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 394319c818..544d45cb9d 100644
--- a/xen/include/xen/lib.h
+++ b/xen/include/xen/lib.h
@@ -61,6 +61,8 @@ debugtrace_printk(const char *fmt, ...) {}
 #define _p(_x) ((void *)(unsigned long)(_x))
 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...)               \
 ({                                              \
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.19