[PATCH] fetch: allow running as different users in shared repositories

Vadim Zeitlin· Mar 19, 2020, 01:03 UTC · lore

The function fopen_for_writing(), which was added in 79d7582e32 (commit: allow editing the commit message even in shared repos, 2016-01-06) and used for overwriting FETCH_HEAD since ea56518dfe (Handle more file writes correctly in shared repos, 2016-01-11), didn't do it correctly in shared repositories under Linux.

This happened because in this situation the file FETCH_HEAD has mode 644 and attempting to overwrite it when running git-fetch under an account different from the one that was had originally created it, failed with EACCES, and not EPERM. However fopen_for_writing() only checked for the latter, and not the former, so it didn't even try removing the existing file and recreating it, as it was supposed to do.

Fix this by checking for either EACCES or EPERM. The latter doesn't seem to be ever returned in a typical situation by open(2) under Linux, but keep checking for it as it is presumably returned under some other platform, although it's not really clear where does this happen.

Signed-off-by: Vadim Zeitlin <vz-git@zeitlins.org>
---
I couldn't find any system that would return EPERM for a "normal"
permissions denied error, so maybe it's not worth checking for it, but I
wanted to minimize the number of changes to the existing behaviour. At the
very least, testing for EACCES is definitely necessary under Linux, where
openat(2) returns it, and not EPERM, in the situation described above, i.e.
non-writable file (even if it's in a writable directory, allowing to unlink
it without problems).
---
 wrapper.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

Show changes to wrapper.c +3 −2

diff --git a/wrapper.c b/wrapper.c
index e1eaef2e16..f5607241da 100644
--- a/wrapper.c
+++ b/wrapper.c
@@ -373,11 +373,12 @@ FILE *fopen_for_writing(const char *path)
 {
 	FILE *ret = fopen(path, "w");
 
-	if (!ret && errno == EPERM) {
+	if (!ret && (errno == EACCES || errno == EPERM)) {
+		int open_error = errno;
 		if (!unlink(path))
 			ret = fopen(path, "w");
 		else
-			errno = EPERM;
+			errno = open_error;
 	}
 	return ret;
 }

-- 
2.26.0.rc2

Johannes Schindelin· Mar 25, 2020, 19:04 UTC · re: Vadim Zeitlin · lore

Re: [PATCH] fetch: allow running as different users in shared repositories

Hi Vadim,

On Thu, 19 Mar 2020, Vadim Zeitlin wrote:

Show 7 quoted lines

> The function fopen_for_writing(), which was added in 79d7582e32 (commit:
> allow editing the commit message even in shared repos, 2016-01-06) and
> used for overwriting FETCH_HEAD since ea56518dfe (Handle more file
> writes correctly in shared repos, 2016-01-11), didn't do it correctly in
> shared repositories under Linux.
>
> This happened because in this situation the file FETCH_HEAD has mode 644

I wonder why that is. In a shared repository, it should have mode 664, I thought.

Show 20 quoted lines

> and attempting to overwrite it when running git-fetch under an account
> different from the one that was had originally created it, failed with
> EACCES, and not EPERM. However fopen_for_writing() only checked for the
> latter, and not the former, so it didn't even try removing the existing
> file and recreating it, as it was supposed to do.
>
> Fix this by checking for either EACCES or EPERM. The latter doesn't seem
> to be ever returned in a typical situation by open(2) under Linux, but
> keep checking for it as it is presumably returned under some other
> platform, although it's not really clear where does this happen.
>
> Signed-off-by: Vadim Zeitlin <vz-git@zeitlins.org>
> ---
> I couldn't find any system that would return EPERM for a "normal"
> permissions denied error, so maybe it's not worth checking for it, but I
> wanted to minimize the number of changes to the existing behaviour. At the
> very least, testing for EACCES is definitely necessary under Linux, where
> openat(2) returns it, and not EPERM, in the situation described above, i.e.
> non-writable file (even if it's in a writable directory, allowing to unlink
> it without problems).

That rationale makes sense to me, as does the patch.

Thanks, Johannes

Show 26 quoted lines

> ---
>  wrapper.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/wrapper.c b/wrapper.c
> index e1eaef2e16..f5607241da 100644
> --- a/wrapper.c
> +++ b/wrapper.c
> @@ -373,11 +373,12 @@ FILE *fopen_for_writing(const char *path)
>  {
>  	FILE *ret = fopen(path, "w");
>
> -	if (!ret && errno == EPERM) {
> +	if (!ret && (errno == EACCES || errno == EPERM)) {
> +		int open_error = errno;
>  		if (!unlink(path))
>  			ret = fopen(path, "w");
>  		else
> -			errno = EPERM;
> +			errno = open_error;
>  	}
>  	return ret;
>  }
> --
> 2.26.0.rc2
>

patchfetch: allow running as different users in shared repositories