linux-kernel - RE: [PATCH] kbuild: use more portable 'command -v' for cc-cross-prefix

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Date:   Mon, 3 Jun 2019 11:14:03 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Masahiro Yamada' <yamada.masahiro@...ionext.com>,
        "linux-kbuild@...r.kernel.org" <linux-kbuild@...r.kernel.org>
CC:     Vineet Gupta <vgupta@...opsys.com>,
        Alexey Brodkin <abrodkin@...opsys.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        linux-stable <stable@...r.kernel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] kbuild: use more portable 'command -v' for
 cc-cross-prefix

From: Masahiro Yamada
> Sent: 03 June 2019 11:49
> 
> 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 scripting, but it is not 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 annoying
> warnings from the 'which' on CentOS 7 when the given command is not
> found 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' is different on environment, I want
> to try 'command -v' 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 line and omitting the subshell will not
> change the behavior.
> 
> In this case, no shell special character is used. So, Make will try
> to run the command directly. However, 'command' is a shell-builtin
> command. In fact, Make has a table of shell-builtin commands because
> it must spawn a subshell 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@....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@...conx.ca>.
> 
> This is not included in any released versions of Make yet.
> (But, some distributions may have back-ported the fix-up.)
> 
> To trick Make and let it fork the subshell, I added a shell special
> character '~'. We may be able to get rid of this workaround someday,
> but it is very far into the future.
> 
> [1] http://pubs.opengroup.org/onlinepubs/9699919799/utilities/command.html
> 
> Fixes: bd55f96fa9fc ("kbuild: refactor cc-cross-prefix implementation")
> Cc: linux-stable <stable@...r.kernel.org> # 5.1
> Reported-by: Alexey Brodkin <abrodkin@...opsys.com>
> Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
> ---
> 
>  scripts/Kbuild.include | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
> index 85d758233483..5a32ca80c3f6 100644
> --- a/scripts/Kbuild.include
> +++ b/scripts/Kbuild.include
> @@ -74,8 +74,11 @@ 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: the special character '~' forces Make to invoke a shell. This workaround
> +# is needed because this issue was only fixed after 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 ~), $(c))))

I see a problem here:
	command -v foo bar
could be deemed to be an error (extra argument).

You could use:
	$(shell sh -c "command -v $(c)gcc")
or maybe:
	$(shell command$${x:+} -v $(c)gcc)

	David

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