Message-Id: <20200910112701.13853-1-linux@rasmusvillemoes.dk>
Date:   Thu, 10 Sep 2020 13:26:57 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Brian Norris <briannorris@...omium.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Bhaskar Chowdhury <unixbhaskar@...il.com>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] scripts/setlocalversion: make git describe output more reliable

When building for an embedded target using Yocto, we're sometimes
observing that the version string that gets built into vmlinux (and
thus what uname -a reports) differs from the path under /lib/modules/
where modules get installed in the rootfs, but only in the length of
the -gabc123def suffix. Hence modprobe always fails.

The problem is that Yocto has the concept of "sstate" (shared state),
which allows different developers/buildbots/etc. to share build
artifacts, based on a hash of all the metadata that went into building
that artifact - and that metadata includes all dependencies (e.g. the
compiler used etc.). That normally works quite well; usually a clean
build (without using any sstate cache) done by one developer ends up
being binary identical to a build done on another host. However, one
thing that can cause two developers to end up with different builds
[and thus make one's vmlinux package incompatible with the other's
kernel-dev package], which is not captured by the metadata hashing, is
this `git describe`: The output of that can be affected by

(1) git version: before 2.11 git defaulted to a minimum of 7, since
2.11 (git.git commit e6c587) the default is dynamic based on the
number of objects in the repo
(2) hence even if both run the same git version, the output can differ
based on how many remotes are being tracked (or just lots of local
development branches or plain old garbage)
(3) and of course somebody could have a core.abbrev config setting in
~/.gitconfig

So in order to avoid `uname -a` output relying on such random details
of the build environment which are rather hard to ensure are
consistent between developers and buildbots, use an explicit
--abbrev=15 option (and for consistency, also use rev-parse --short=15
for the unlikely case of no signed tags being usable).

Now, why is 60 bits enough for everyone? It's not mathematically
guaranteed that git won't have to use 16 in some git repo, but it is
beyond unlikely: Even in a repo with 100M objects, the probability
that any given commit (i.e. the one being described) clashes with some
other object in the first 15 hex chars is less than 1e-10, and
currently a git repo tracking Linus', -stable and -rt only has around
10M objects.

Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
I could probably fix things by adding a 'git config --local
core.abbrev 15' step to the Yocto build process after the repo to
build from has been cloned but before building has started. But in the
interest of binary reproducibility outside of just Yocto builds, I
think it's better if this lives in the kernel.

 scripts/setlocalversion | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/scripts/setlocalversion b/scripts/setlocalversion
index 20f2efd57b11..c5262f0d953d 100755
--- a/scripts/setlocalversion
+++ b/scripts/setlocalversion
@@ -45,7 +45,7 @@ scm_version()
 
 	# Check for git and a git repo.
 	if test -z "$(git rev-parse --show-cdup 2>/dev/null)" &&
-	   head=$(git rev-parse --verify --short HEAD 2>/dev/null); then
+	   head=$(git rev-parse --verify --short=15 HEAD 2>/dev/null); then
 
 		# If we are at a tagged commit (like "v2.6.30-rc6"), we ignore
 		# it, because this version is defined in the top level Makefile.
@@ -59,7 +59,7 @@ scm_version()
 			fi
 			# If we are past a tagged commit (like
 			# "v2.6.30-rc5-302-g72357d5"), we pretty print it.
-			if atag="$(git describe 2>/dev/null)"; then
+			if atag="$(git describe --abbrev=15 2>/dev/null)"; then
 				echo "$atag" | awk -F- '{printf("-%05d-%s", $(NF-1),$(NF))}'
 
 			# If we don't have a tag at all we print -g{commitish}.
-- 
2.23.0

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