• Masahiro Yamada's avatar
    kbuild: use more portable 'command -v' for cc-cross-prefix · 913ab978
    Masahiro Yamada authored
    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 bd55f96f ("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: bd55f96f ("kbuild: refactor cc-cross-prefix implementation")
    Cc: linux-stable <stable@vger.kernel.org> # 5.1
    Reported-by: default avatarAlexey Brodkin <abrodkin@synopsys.com>
    Signed-off-by: default avatarMasahiro Yamada <yamada.masahiro@socionext.com>
    Tested-by: default avatarAlexey Brodkin <abrodkin@synopsys.com>
    913ab978
Kbuild.include 12.7 KB