[PATCH v3] scripts/check-local-export: avoid 'wait $!' for process substitution

[Masahiro Yamada <masahiroy@kernel.org>]

Bash 4.4, released in 2016, supports 'wait $!' to check the exit status
of a process substitution, but it seems too new.

Some people using older bash versions (on CentOS 7, Ubuntu 16.04, etc.)
reported an error like this:

  ./scripts/check-local-export: line 54: wait: pid 17328 is not a child of this shell

I used the process substitution to avoid a pipeline, which executes each
command in a subshell. If the while-loop is executed in the subshell
context, variable changes within are lost after the subshell terminates.

Fortunately, Bash 4.2, released in 2011, supports the 'lastpipe' option,
which runs the last element of a pipeline in the current shell process.

Also, set 'pipefail' to catch errors from ${NM}.

Add the bash requirement to Documentation/process/changes.rst.

Fixes: 31cb50b5590f ("kbuild: check static EXPORT_SYMBOL* by script instead of modpost")
Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Reported-by: Wang Yugui <wangyugui@e16-tech.com>
Tested-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Tested-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---

Changes in v3:
  - Add "TODO:" to the comment block (Nick)

Changes in v2:
  - Add bash to Documentation/process/changes.rst

 Documentation/process/changes.rst | 12 +++++++++++
 scripts/check-local-export        | 36 ++++++++++++++++++-------------
 2 files changed, 33 insertions(+), 15 deletions(-)

diff --git a/Documentation/process/changes.rst b/Documentation/process/changes.rst
index 34415ae1af1b..19c286c23786 100644
--- a/Documentation/process/changes.rst
+++ b/Documentation/process/changes.rst
@@ -32,6 +32,7 @@ you probably needn't concern yourself with pcmciautils.
 GNU C                  5.1              gcc --version
 Clang/LLVM (optional)  11.0.0           clang --version
 GNU make               3.81             make --version
+bash                   4.2              bash --version
 binutils               2.23             ld -v
 flex                   2.5.35           flex --version
 bison                  2.0              bison --version
@@ -84,6 +85,12 @@ Make
 
 You will need GNU make 3.81 or later to build the kernel.
 
+Bash
+----
+
+Some bash scripts are used for the kernel build.
+Bash 4.2 or newer is needed.
+
 Binutils
 --------
 
@@ -362,6 +369,11 @@ Make
 
 - <ftp://ftp.gnu.org/gnu/make/>
 
+Bash
+----
+
+- <ftp://ftp.gnu.org/gnu/bash/>
+
 Binutils
 --------
 
diff --git a/scripts/check-local-export b/scripts/check-local-export
index da745e2743b7..6ccc2f467416 100755
--- a/scripts/check-local-export
+++ b/scripts/check-local-export
@@ -8,11 +8,31 @@
 
 set -e
 
+# catch errors from ${NM}
+set -o pipefail
+
+# Run the last element of a pipeline in the current shell.
+# Without this, the while-loop would be executed in a subshell, and
+# the changes made to 'symbol_types' and 'export_symbols' would be lost.
+shopt -s lastpipe
+
 declare -A symbol_types
 declare -a export_symbols
 
 exit_code=0
 
+# If there is no symbol in the object, ${NM} (both GNU nm and llvm-nm) shows
+# 'no symbols' diagnostic (but exits with 0). It is harmless and hidden by
+# '2>/dev/null'. However, it suppresses real error messages as well. Add a
+# hand-crafted error message here.
+#
+# TODO:
+# Use --quiet instead of 2>/dev/null when we upgrade the minimum version of
+# binutils to 2.37, llvm to 13.0.0.
+# Then, the following line will be really simple:
+#   ${NM} --quiet ${1} |
+
+{ ${NM} ${1} 2>/dev/null || { echo "${0}: ${NM} failed" >&2; false; } } |
 while read value type name
 do
 	# Skip the line if the number of fields is less than 3.
@@ -37,21 +57,7 @@ do
 	if [[ ${name} == __ksymtab_* ]]; then
 		export_symbols+=(${name#__ksymtab_})
 	fi
-
-	# If there is no symbol in the object, ${NM} (both GNU nm and llvm-nm)
-	# shows 'no symbols' diagnostic (but exits with 0). It is harmless and
-	# hidden by '2>/dev/null'. However, it suppresses real error messages
-	# as well. Add a hand-crafted error message here.
-	#
-	# Use --quiet instead of 2>/dev/null when we upgrade the minimum version
-	# of binutils to 2.37, llvm to 13.0.0.
-	#
-	# Then, the following line will be really simple:
-	#   done < <(${NM} --quiet ${1})
-done < <(${NM} ${1} 2>/dev/null || { echo "${0}: ${NM} failed" >&2; false; } )
-
-# Catch error in the process substitution
-wait $!
+done
 
 for name in "${export_symbols[@]}"
 do
-- 
2.32.0