On Mon, Feb 8, 2021 at 10:48 PM David Laight [off-list ref] wrote:

From: Sasha Levin

quoted

Sent: 06 February 2021 03:51

Right now if SUBLEVEL becomes larger than 255 it will overflow into the
territory of PATCHLEVEL, causing havoc in userspace that tests for
specific kernel version.

While userspace code tests for MAJOR and PATCHLEVEL, it doesn't test
SUBLEVEL at any point as ABI changes don't happen in the context of
stable tree.

Thus, to avoid overflows, simply clamp SUBLEVEL to it's maximum value in
the context of LINUX_VERSION_CODE. This does not affect "make
kernelversion" and such.

Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 Makefile | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/Makefile b/Makefile
index 49ac1b7fe8e99..157be50c691e5 100644
--- a/Makefile
+++ b/Makefile

@@ -1258,9 +1258,15 @@ define filechk_utsrelease.h
 endef

 define filechk_version.h
-     echo \#define LINUX_VERSION_CODE $(shell                         \
-     expr $(VERSION) \* 65536 + 0$(PATCHLEVEL) \* 256 + 0$(SUBLEVEL)); \
-     echo '#define KERNEL_VERSION(a,b,c) (((a) << 16) + ((b) << 8) + (c))'
+     if [ $(SUBLEVEL) -gt 255 ]; then                                 \
+             echo \#define LINUX_VERSION_CODE $(shell                 \
+             expr $(VERSION) \* 65536 + 0$(PATCHLEVEL) \* 256 + 255); \
+     else                                                             \
+             echo \#define LINUX_VERSION_CODE $(shell                 \
+             expr $(VERSION) \* 65536 + 0$(PATCHLEVEL) \* 256 + $(SUBLEVEL)); \
+     fi;                                                              \
+     echo '#define KERNEL_VERSION(a,b,c) (((a) << 16) + ((b) << 8) +  \
+     ((c) > 255 ? 255 : (c)))'
 endef

Why not use KERNEL_VERSION to define LINUX_VERSION_CODE ?
Basically just:
        echo '#define LINUX_VERSION_CODE KERNEL_VERSION($(VERSION), $(PATCHLEVEL)+0, $(SUBLEVEL)+0)'

It was not possible to macrofy LINUX_VERSION_CODE.
(build error when CONFIG_KALLSYMS is disabled)


Presumably, this restriction will go away
with the following commit in linux-next.



commit e06af0b2ba02fc0cc2219a14c4c04ff0296a6f9f
Author: Masahiro Yamada [off-list ref]
Date:   Thu Jan 28 18:42:30 2021 +1100

    init/version.c: remove Version_<LINUX_VERSION_CODE> symbol





My plan is to refactor LINUX_VERSION_CODE in the next
development cycle.

If PATCHLEVEL and SUBLEVEL are guaranteed to be non-empty the +0
can be removed.
The patch assumes they are non-empty, the original pre-prended 0
to stop syntax error for empty version strings.

Note that the expr version will process 08 and 09.
gcc will treat them as octal, and may error them.

        David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

--
Best Regards
Masahiro Yamada