[PATCH for-4.20(?) 1/3] xen/console: Fix truncation of panic() messages

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Julien Grall <julien@xxxxxxx>
CC: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
CC: Bertrand Marquis <bertrand.marquis@xxxxxxx>
CC: Michal Orzel <michal.orzel@xxxxxxx>
CC: Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx>
CC: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>

This taken from a security series (hence partially reviewed already), and
thought it only applied to a forthcoming selftest, but then Ayan posted a
truncated example to Matrix.

Therefore this needs backporting, and might be wanted for 4.20 at this point.
It's low risk and easy to test.
---
 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 7da8c5296f3b..2926c97df9a4 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 81b722ea3e80..130f96791f75 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...)               \
 ({                                              \
-- 
2.39.5