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

[Masahiro Yamada]

Hi David,

On Mon, Jun 3, 2019 at 8:14 PM David Laight <David.Laight@xxxxxxxxxx> wrote:
>
> 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@xxxxxxx>
> > | 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@xxxxxxxxxx>.
> >
> > 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@xxxxxxxxxxxxxxx> # 5.1
> > Reported-by: Alexey Brodkin <abrodkin@xxxxxxxxxxxx>
> > Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
> > ---
> >
> >  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).

OK, the specification does not allow to pass arguments
with -v.


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


How about this?

          $(shell : ~; command -v $(c)gcc)



-- 
Best Regards
Masahiro Yamada