Re: Bug: git branch --unset-upstream command can nuke config when disk is full.

On Wed, Sep 13, 2017 at 05:18:56PM +0200, demerphq wrote:

quoted

Hmph. That is very disturbing. But with that information I should be
able to track down the culprit. Thanks for digging.

FWIW, I see that git_config_set_multivar_in_file_gently() uses
write_in_full() which in turn uses xwrite(), but the latter has the
following comment on it:

/*
 * xwrite() is the same a write(), but it automatically restarts write()
 * operations with a recoverable error (EAGAIN and EINTR). xwrite() DOES NOT
 * GUARANTEE that "len" bytes is written even if the operation is successful.
 */

I suspect that at this point I am not adding much value here, so I
will leave it at this.

No, the problem is in this line:

                                 if (write_in_full(fd, contents + copy_begin,
                                                   copy_end - copy_begin) <
                                     copy_end - copy_begin)
                                          goto write_err_out;

write_in_full() returns -1 on error (_not_ how many bytes were actually
written). So its return is a signed ssize_t. But the result of the
pointer subtraction "copy_end - copy_begin" is an unsigned ptrdiff_t.
The compiler promotes the signed to an unsigned, so the condition can
never be true (the "-1" becomes the highest possible value).

I have the fix, but I'm searching the code base for other instances of
the same error.

-Peff