[PATCH 3/3] compat: make compat_sysinfo() scale far enough to actually fit

[Bradley Morgan <brads@mainlining.org>]

compat_sysinfo() checks whether totalram or totalswap is too big for the
compat struct and scales the values down to fit, but the loop stops as
soon as s.mem_unit reaches PAGE_SIZE.  That caps the compression at one
page, 12 bits with 4K pages.  On a 64 bit kernel do_sysinfo() always
hands it values in bytes with mem_unit == 1, its own overflow math runs
on unsigned long so it does not bail until 16 EiB, so the loop runs its
12 turns, the values come back out as a page count, and anything past
2^32 pages still does not fit.  Storing that into the u32 field then
just drops the top bits.

So a 32 bit process on a box past 2^32 pages gets a wrapped number
instead of a coarse one, with 4K pages that starts at 16 TiB:

  ram      old                  new
  8 TiB    2147483648 x 4096    unchanged
  16 TiB   0 x 4096             2147483648 x 8192
  17 TiB   268435456 x 4096     2281701376 x 8192
  32 TiB   0 x 4096             2147483648 x 16384

The 17 TiB row is the nasty one, a 32 bit process is told the box has
1 TiB of ram.  Keep scaling until the values fit instead of until the
unit reaches a page, the unit then says how coarse the number is, which
is what mem_unit is for.

Nothing that works today changes.  I ran both loops side by side over
ram from 1 MiB to 2^56 with 4K and 64K pages, they disagree on exactly
the inputs where the old one dropped bits and agree on every one it got
right.

Signed-off-by: Bradley Morgan <brads@mainlining.org>
---
 kernel/sys.c | 26 ++++++++++++--------------
 1 file changed, 12 insertions(+), 14 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index c4839c986f5e0..8f683b6bb87b8 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -3035,24 +3035,22 @@ COMPAT_SYSCALL_DEFINE1(sysinfo, struct compat_sysinfo __user *, info)
 	do_sysinfo(&s);
 
 	/* Check to see if any memory value is too large for 32-bit and scale
-	 *  down if needed
+	 * down if needed.  Keep going until the values fit, not just until the
+	 * unit reaches a page.  The values halve each turn, so this ends.
 	 */
 	if (upper_32_bits(s.totalram) || upper_32_bits(s.totalswap)) {
-		int bitcount = 0;
-
-		while (s.mem_unit < PAGE_SIZE) {
+		while (s.mem_unit < PAGE_SIZE || upper_32_bits(s.totalram) ||
+		       upper_32_bits(s.totalswap)) {
 			s.mem_unit <<= 1;
-			bitcount++;
+			s.totalram >>= 1;
+			s.freeram >>= 1;
+			s.sharedram >>= 1;
+			s.bufferram >>= 1;
+			s.totalswap >>= 1;
+			s.freeswap >>= 1;
+			s.totalhigh >>= 1;
+			s.freehigh >>= 1;
 		}
-
-		s.totalram >>= bitcount;
-		s.freeram >>= bitcount;
-		s.sharedram >>= bitcount;
-		s.bufferram >>= bitcount;
-		s.totalswap >>= bitcount;
-		s.freeswap >>= bitcount;
-		s.totalhigh >>= bitcount;
-		s.freehigh >>= bitcount;
 	}
 
 	memset(&s_32, 0, sizeof(s_32));
-- 
2.53.0