Re: [PATCH] strbuf: support long paths w/o read rights in strbuf_getcwd() on FreeBSD

[Christian Couder <christian.couder@gmail.com>]

On Tue, Mar 28, 2017 at 11:49 PM, Jeff King <peff@peff.net> wrote:
> On Tue, Mar 28, 2017 at 11:15:12PM +0200, Christian Couder wrote:
>
>> On Sun, Mar 26, 2017 at 3:43 PM, René Scharfe <l.s.r@web.de> wrote:
>> > FreeBSD implements getcwd(3) as a syscall, but falls back to a version
>> > based on readdir(3) if it fails for some reason.  The latter requires
>> > permissions to read and execute path components, while the former does
>> > not.  That means that if our buffer is too small and we're missing
>> > rights we could get EACCES, but we may succeed with a bigger buffer.
>> >
>> > Keep retrying if getcwd(3) indicates lack of permissions until our
>> > buffer can fit PATH_MAX bytes, as that's the maximum supported by the
>> > syscall on FreeBSD anyway.  This way we do what we can to be able to
>> > benefit from the syscall, but we also won't loop forever if there is a
>> > real permission issue.
>>
>> Sorry to be late and maybe I missed something obvious, but the above
>> and the patch seem complex to me compared with something like:
>>
>> diff --git a/strbuf.c b/strbuf.c
>> index ace58e7367..25eadcbedc 100644
>> --- a/strbuf.c
>> +++ b/strbuf.c
>> @@ -441,7 +441,7 @@ int strbuf_readlink(struct strbuf *sb, const char
>> *path, size_t hint)
>>  int strbuf_getcwd(struct strbuf *sb)
>>  {
>>         size_t oldalloc = sb->alloc;
>> -       size_t guessed_len = 128;
>> +       size_t guessed_len = PATH_MAX > 128 ? PATH_MAX : 128;
>>
>>         for (;; guessed_len *= 2) {
>>                 strbuf_grow(sb, guessed_len);
>
> I think the main reason is just that we do not have to pay the price to
> allocate PATH_MAX-sized buffers when they are rarely used.
>
> I doubt it matters all that much in practice, though.

Yeah, I can understand that.

But I also wonder if René's patch relies on PATH_MAX being a multiple
of 128 on FreeBSD and what would happen for a path between 129 and
PATH_MAX if PATH_MAX was something like 254.