On Wed, Sep 13, 2017 at 10:47:28AM -0700, Jonathan Nieder wrote: > Jeff King wrote: > > > I scoured the code base for cases of this, but it turns out > > that these two in git_config_set_multivar_in_file_gently() > > are the only ones. This case is actually quite interesting: > > we don't have a size_t, but rather use the subtraction of > > two pointers. Which you might think would be a signed > > ptrdiff_t, but clearly both gcc and clang treat it as > > unsigned (possibly because the conditional just above > > guarantees that the result is greater than zero). > > Do you have more detail about this? I get worried when I read > something like this that sounds like a compiler bug. > > C99 sayeth: > > When two pointers are subtracted, both shall point to elements > of the same array object, or one past the last element of the > array object; the result is the difference of the subscripts > of the two array elements. The size of the result is > implementation-defined, and its type (a signed integer type) > is ptrdiff_t defined in the <stddef.h> header.
I'm not sure if it's a compiler bug or not. I read the bits about ptrdiff_t, and it wasn't entirely clear to me if a pointer difference _is_ an actual ptrdiff_t, or if it can generally be stored in one. Right below that text it also says: If the result is not representable in an object of that type, the behavior is undefined. That said, I might be wrong that unsigned promotion is the culprit. I didn't look at the generated assembly. But I also can't see what else would be causing the problem here. We're clearly returning "-1" and the condition doesn't trigger. > How can I reproduce the problem? I gave a recipe in the commit message, which is the best I came up with. You could probably use a fault-injection library to convince write() to fail. Or just tweak the source code to have write_in_full() return -1. > > There's no addition to the test suite here, since you need > > to convince write() to fail in order to see the problem. The > > simplest reproduction recipe I came up with is to trigger > > ENOSPC (this only works on Linux, obviously): > > Does /dev/full make it simpler to reproduce? I don't think so, because the write() failure is to the lockfile, which is created with O_EXCL. So even if you could convince "config.lock" to be the right device type, the open() would fail. -Peff
