Re: GIT_ASKPASS absolute path detection bug on Windows

["András Kucsma" <andras.kucsma@gmail.com>]

On Sun, Mar 22, 2020 at 8:31 AM Torsten Bögershausen <tboegi@web.de> wrote:
>
> On Sat, Mar 21, 2020 at 12:42:50PM +0100, András Kucsma wrote:
> > Hi All,
> >
> > I believe to have found an issue regarding properly executing the
> > GIT_ASKPASS binary. I'm using Windows Server 2019, with git 2.21.0
> > installed using cygwin.
> >
> > ## To reproduce:
> >
> > Assume you have the askpass binary at C:\askpass.bat. In CMD the
> > following commands reproduce the issue:
> >
> > C:\> set GIT_ASKPASS=C:\askpass.bat
> > C:\> git clone https://<private_repository>.git
> > Cloning into '<private_repository>'...
> > error: cannot run C:\askpass.bat: No such file or directory
> > [... proceeds to interactively ask for username and password ...]
> >
> > On the other hand, if we change the GIT_ASKPASS environment variable
> > slightly, so that there is a forward slash (/) instead of a backslash
> > (\), things work as expected:
> >
> > C:\> set GIT_ASKPASS=C:/askpass.bat
> > C:\> git clone https://<private_repository>.git
> > Cloning into '<private_repository>'...
> > [... success ...]
> >
> > ## Some context:
> >
> > The source of the problem, is that if git doesn't find a forward slash
> > anywhere in the path, it assumes it is not a real path and has to look
> > for the binary using the PATH environment variable. See in
> > prepare_cmd():
> > https://github.com/git/git/blob/98cedd0233e/run-command.c#L429-L439
> >
> > You can see that the "cannot run" error message is printed here, just
> > after prepare_cmd() returned -1:
> > https://github.com/git/git/blob/98cedd0233e/run-command.c#L749-L753
> >
> > I believe this was introduced in late 2018 around git v2.19.2,
> > although I did not actually bisect the issue:
> > https://github.com/git/git/commit/321fd823897#diff-7577a5178f8cdc0f719e580577889f04R401-R415
> >
> >
> > I hope I'm sharing this bug at the right forum. Please direct me to
> > the proper place if not.
>
> Yes, you came to the rigth place.
> Thanks for the report and the detailed analysis.
>
> A quick fix, and a begin of a patch, could be to use
> has_dos_drive_prefix() which will look for C: and will therefore even work
> with C:\
>
>         /*
>          * If there are no '/' characters in the command then perform a path
>          * lookup and use the resolved path as the command to exec.  If there
>          * are '/' characters, we have exec attempt to invoke the command
>          * directly.
>          */
>         if ((!strchr(out->argv[1], '/')) ||
>             (has_dos_drive_prefix(out->argv[1]))) {
>                 char *program = locate_in_PATH(out->argv[1]);
> []
>
> If you want to play around with the code a little bit, and send us a "git diff",
> we can convert that into a patch.
>
> Wellcome to the Git community.
>
> >
> > Thank you,
> > Andras

Thanks Torsten!

I believe it is not enough to test only for the drive specifier, as
GIT_ASKPASS has to work with relative paths as well:
C:\SomeDirectory> set GIT_ASKPASS=.\SomeOtherDirectory\askpass.bat
C:\SomeDirectory> git clone https://<some_private_repository>.git

My proposal patch is to take advantage of find_last_dir_sep function's
OS specific directory separator knowledge.
I posted the diff below, which is also available on github here:
https://github.com/git/git/compare/maint...r0mai:fix-prepare_cmd-windows-maint

diff --git a/run-command.c b/run-command.c
index f5e1149f9b..9fcc12ebf9 100644
--- a/run-command.c
+++ b/run-command.c
@@ -421,12 +421,12 @@ static int prepare_cmd(struct argv_array *out,
const struct child_process *cmd)
     }

     /*
-     * If there are no '/' characters in the command then perform a path
-     * lookup and use the resolved path as the command to exec.  If there
-     * are '/' characters, we have exec attempt to invoke the command
-     * directly.
+     * If there are no dir separator characters in the command then perform
+     * a path lookup and use the resolved path as the command to exec. If
+     * there are dir separator characters, we have exec attempt to invoke
+     * the command directly.
      */
-    if (!strchr(out->argv[1], '/')) {
+    if (find_last_dir_sep(out->argv[1]) == NULL) {
         char *program = locate_in_PATH(out->argv[1]);
         if (program) {
             free((char *)out->argv[1]);