From: Masahiro Yamada yamada.masahiro@socionext.com
commit 913ab9780fc021298949cc5514d6255a008e69f9 upstream.
To print the pathname that will be used by shell in the current environment, 'command -v' is a standardized way. [1]
'which' is also often used in scripts, but it is less portable.
When I worked on commit bd55f96fa9fc ("kbuild: refactor cc-cross-prefix implementation"), I was eager to use 'command -v' but it did not work. (The reason is explained below.)
I kept 'which' as before but got rid of '> /dev/null 2>&1' as I thought it was no longer needed. Sorry, I was wrong.
It works well on my Ubuntu machine, but Alexey Brodkin reports noisy warnings on CentOS7 when 'which' fails to find the given command in the PATH environment.
$ which foo which: no foo in (/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin)
Given that behavior of 'which' depends on system (and it may not be installed by default), I want to try 'command -v' once again.
The specification [1] clearly describes the behavior of 'command -v' when the given command is not found:
Otherwise, no output shall be written and the exit status shall reflect that the name was not found.
However, we need a little magic to use 'command -v' from Make.
$(shell ...) passes the argument to a subshell for execution, and returns the standard output of the command.
Here is a trick. GNU Make may optimize this by executing the command directly instead of forking a subshell, if no shell special characters are found in the command and omitting the subshell will not change the behavior.
In this case, no shell special character is used. So, Make will try to run it directly. However, 'command' is a shell-builtin command, then Make would fail to find it in the PATH environment:
$ make ARCH=m68k defconfig make: command: Command not found make: command: Command not found make: command: Command not found
In fact, Make has a table of shell-builtin commands because it must ask the shell to execute them.
Until recently, 'command' was missing in the table.
This issue was fixed by the following commit:
| commit 1af314465e5dfe3e8baa839a32a72e83c04f26ef | Author: Paul Smith psmith@gnu.org | Date: Sun Nov 12 18:10:28 2017 -0500 | | * job.c: Add "command" as a known shell built-in. | | This is not a POSIX shell built-in but it's common in UNIX shells. | Reported by Nick Bowler nbowler@draconx.ca.
Because the latest release is GNU Make 4.2.1 in 2016, this commit is not included in any released versions. (But some distributions may have back-ported it.)
We need to trick Make to spawn a subshell. There are various ways to do so:
1) Use a shell special character '~' as dummy
$(shell : ~; command -v $(c)gcc)
2) Use a variable reference that always expands to the empty string (suggested by David Laight)
$(shell command$${x:+} -v $(c)gcc)
3) Use redirect
$(shell command -v $(c)gcc 2>/dev/null)
I chose 3) to not confuse people. The stderr would not be polluted anyway, but it will provide extra safety, and is easy to understand.
Tested on Make 3.81, 3.82, 4.0, 4.1, 4.2, 4.2.1
[1] http://pubs.opengroup.org/onlinepubs/9699919799/utilities/command.html
Fixes: bd55f96fa9fc ("kbuild: refactor cc-cross-prefix implementation") Cc: linux-stable stable@vger.kernel.org # 5.1 Reported-by: Alexey Brodkin abrodkin@synopsys.com Signed-off-by: Masahiro Yamada yamada.masahiro@socionext.com Tested-by: Alexey Brodkin abrodkin@synopsys.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- scripts/Kbuild.include | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-)
--- a/scripts/Kbuild.include +++ b/scripts/Kbuild.include @@ -73,8 +73,13 @@ endef # Usage: CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu- m68k-linux-) # Return first <prefix> where a <prefix>gcc is found in PATH. # If no gcc found in PATH with listed prefixes return nothing +# +# Note: '2>/dev/null' is here to force Make to invoke a shell. Otherwise, it +# would try to directly execute the shell builtin 'command'. This workaround +# should be kept for a long time since this issue was fixed only after the +# GNU Make 4.2.1 release. cc-cross-prefix = $(firstword $(foreach c, $(filter-out -%, $(1)), \ - $(if $(shell which $(c)gcc), $(c)))) + $(if $(shell command -v $(c)gcc 2>/dev/null), $(c))))
# output directory for tests below TMPOUT := $(if $(KBUILD_EXTMOD),$(firstword $(KBUILD_EXTMOD))/)