RFR: 8229872: (fs) Increase buffer size used with getmntent

[Florian Weimer]

* Vladimir Kempik:

> Please review the suggested fix for bug JDK-8229872
>
> Files.getFileStore() will fail if there is an entry in /proc/mounts which is greater than 1024 bytes length
> This happens because we allocate 1024 bytes size buffer on stack for use with getmntent_r.
>
> A possible way is to use getmntent instead, it will allocated 4kb buffer on glibc and «as much as needed» on musl
> (https://www.openwall.com/lists/musl/2016/09/08/1)
> But getmntent is dubbed "MT-Unsafe race:mntentbuf locale»
>
> I have tested this fix on openjdk8u222 and it can parse long entries fine.

I think the bug still persists because a mount path can be up to 4095
bytes long, and it can be expanded to four times that using octal
escapes.

getmntent really isn't thread-safe in the current glibc implementation,
so I don't think you can use that.

I think the easiest solution will be to parse /proc/mounts directly,
from Java code.  The file format is expected to be stable and quite
simple.  The code probably won't be much longer than for parsing struct
mntent. 8-/

In the future, there might be an RFE to ignore certain entries (e.g.,
those installed by autofs), but the kernel doesn't currently provide the
flag required for that.

Thanks,
Florian