RFR: 8273111: Default timezone should return zone ID if /etc/localtime is valid but not canonicalization on linux

[Wu Yan]

On Wed, 1 Sep 2021 12:51:15 GMT, Florian Weimer <fweimer at openjdk.org> wrote:

>> Hi,
>> Please help me review the change to enhance getting  time zone ID from /etc/localtime on linux.
>> 
>> We use `realpath` instead of `readlink` to obtain the link name of /etc/localtime, because `readlink` can only read the value of a symbolic of link, not the canonicalized absolute pathname.
>> 
>> For example, the value of /etc/localtime is "../usr/share/zoneinfo//Asia/Shanghai", then the linkbuf obtained by `readlink` is "../usr/share/zoneinfo//Asia/Shanghai", and then the call of `getZoneName(linkbuf)` will get "/Asia/Shanghai", not "Asia/Shanghai", which consider as invalid in `ZoneInfoFile.getZoneInfo()`. Using `realpath`, you can get “/usr/share/zoneinfo/Asia/Shanghai“ directly from “/etc/localtime“.
>> 
>> Thanks,
>> wuyan
>
> src/java.base/unix/native/libjava/TimeZone_md.c line 292:
> 
>> 290:         /* canonicalize the path */
>> 291:         char resolvedpath[PATH_MAX + 1];
>> 292:         char *path = realpath(DEFAULT_ZONEINFO_FILE, resolvedpath);
> 
> You really should use `realpath` with `NULL` as the second argument, to avoid any risk of buffer overflow. Future C library headers may warn about non-null arguments here.

Ok, Thanks. I will refer to the implementation of `os::Posix::realpath` to fix it if we still use `realpath`. https://github.com/openjdk/jdk/blob/5245c1cf0260a78ca5f8ab4e7d13107f21faf071/src/hotspot/os/posix/os_posix.cpp#L805

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PR: https://git.openjdk.java.net/jdk/pull/5327