[RFC PATCH 1/2] log2: handle LARGE input to __roundup_pow_of_two()

[Randy Dunlap]

UBSAN detected a 64-bit shift in log2.h:__roundup_pow_of_two():
  UBSAN: shift-out-of-bounds in ./include/linux/log2.h:57:13
  shift exponent 64 is too large for 64-bit type 'long unsigned int'

This is during a call from mm/readahead.c:ondemand_readahead(),
get_init_ra_size(), where the 'size' parameter must have been
extremely large (or "negative").

fls() can legitimately return 32 or 64 when the MSbit is set in
a 32-bit or 64-bit unsigned long. For these values, doing
"1UL << shiftcout" is invalid or undefined, so catch when this
happens.

When the MSbit is 32 or 64, we cannot roundup to the next power of 2,
so just return n (the input value), which is >= 0x8000...0000 and
probably not a power of 2 (unless it is exactly 0x8000...0000).

Signed-off-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Toralf Förster <toralf.foerster@xxxxxx>
Cc: linux-mm@xxxxxxxxx
---
 include/linux/log2.h |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

--- linux-5.10.1.orig/include/linux/log2.h
+++ linux-5.10.1/include/linux/log2.h
@@ -54,7 +54,17 @@ bool is_power_of_2(unsigned long n)
 static inline __attribute__((const))
 unsigned long __roundup_pow_of_two(unsigned long n)
 {
-	return 1UL << fls_long(n - 1);
+	unsigned int lastset = fls_long(n - 1); /* this can be 64 or 32 */
+
+	/*
+	 * for high bit set (64 or 32), we can neither round up nor
+	 * make it a power or 2
+	 */
+	if ((sizeof(n) == 4 && lastset == 32) ||
+	    (sizeof(n) == 8 && lastset == 64))
+		return n;
+
+	return 1UL << lastset;
 }
 
 /**