The following issue came up during the code review of the following bug:

    JDK-8005044 remove crufty '_g' support from HS runtime code

This question:

> Another nit: Is it worth checking that all of
> "/hotspot/libjvm.so" could get copied to buf? (i.e.: that
> buflen-len > strlen("/hotspot/libjvm.so")

is straight forward and the short answer is: yes, that
code makes assumptions that "/hotspot/libjvm.so" will
fit into 'buf'. It should not do that.

It is Ron's call as to whether he wants to fix this existing
bug as part of this work. It is easy enough to fix the obvious
part of the bug, but... as is normal with HotSpot, there
is more going on here than meets the eye...

The slightly longer answer is that it probably doesn't
matter because the path being created here is a fake.
As long as the "/hotspot/l" part gets copied, then the
callers of os::jvm_path() will be able to handle the
fakery.

Of course, there is an even longer answer. Read on if
you want gory details (and a history lesson).

This gamma launcher stuff is giving me an absolute headache.
Unfortunately, there is a lot of history involved here...

Here are the variations:

src/os/bsd/vm/os_bsd.cpp:

   1715 if (Arguments::created_by_gamma_launcher()) {
   1716 // Support for the gamma launcher. Typical value for buf is
   1717 // "<JAVA_HOME>/jre/lib/<arch>/<vmtype>/libjvm". If "/jre/lib/" appears at
   1718 // the right place in the string, then assume we are installed in a JDK and
   1719 // we're done. Otherwise, check for a JAVA_HOME environment variable and
   1720 // construct a path to the JVM being overridden.

<snip>

   1762 // If the path exists within JAVA_HOME, add the JVM library name
   1763 // to complete the path to JVM being overridden. Otherwise fallback
   1764 // to the path to the current library.
   1765 if (0 == access(buf, F_OK)) {
   1766 // Use current module name "libjvm"
   1767 len = strlen(buf);
   1768 snprintf(buf + len, buflen-len, "/libjvm%s", JNI_LIB_SUFFIX);
   1769 } else {
   1770 // Fall back to path of current library
   1771 rp = realpath(dli_fname, buf);
   1772 if (rp == NULL)
   1773 return;
   1774 }

src/os/linux/vm/os_linux.cpp:

   2206 if (Arguments::created_by_gamma_launcher()) {
   2207 // Support for the gamma launcher. Typical value for buf is
   2208 // "<JAVA_HOME>/jre/lib/<arch>/<vmtype>/libjvm.so". If "/jre/lib/" appears at
   2209 // the right place in the string, then assume we are installed in a JDK and
   2210 // we're done. Otherwise, check for a JAVA_HOME environment variable and fix
   2211 // up the path so it looks like libjvm.so is installed there (append a
   2212 // fake suffix hotspot/libjvm.so).

<snip>

   2243 if (0 == access(buf, F_OK)) {
   2244 // Use current module name "libjvm.so"
   2245 len = strlen(buf);
   2246 snprintf(buf + len, buflen-len, "/hotspot/libjvm.so");
   2247 } else {
   2248 // Go back to path of .so
   2249 rp = realpath(dli_fname, buf);
   2250 if (rp == NULL)
   2251 return;
   2252 }

src/os/solaris/vm/os_solaris.cpp:

   2496 if (Arguments::created_by_gamma_launcher()) {
   2497 // Support for the gamma launcher. Typical value for buf is
   2498 // "<JAVA_HOME>/jre/lib/<arch>/<vmtype>/libjvm.so". If "/jre/lib/" appears at
   2499 // the right place in the string, then assume we are installed in a JDK and
   2500 // we're done. Otherwise, check for a JAVA_HOME environment variable and fix
   2501 // up the path so it looks like libjvm.so is installed there (append a
   2502 // fake suffix hotspot/libjvm.so).

<snip>

   2539 if (0 == access(buf, F_OK)) {
   2540 // Use current module name "libjvm.so"
   2541 len = strlen(buf);
   2542 snprintf(buf + len, buflen-len, "/hotspot/libjvm.so");
   2543 } else {
   2544 // Go back to path of .so
   2545 realpath((char *)dlinfo.dli_fname, buf);
   2546 }

src/os/windows/vm/os_windows.cpp:

   1732 buf[0] = '\0';
   1733 if (Arguments::created_by_gamma_launcher()) {
   1734 // Support for the gamma launcher. Check for an
   1735 // JAVA_HOME environment variable
   1736 // and fix up the path so it looks like
   1737 // libjvm.so is installed there (append a fake suffix
   1738 // hotspot/libjvm.so).
   1739 char* java_home_var = ::getenv("JAVA_HOME");
   1740 if (java_home_var != NULL && java_home_var[0] != 0) {
   1741
   1742 strncpy(buf, java_home_var, buflen);
   1743
   1744 // determine if this is a legacy image or modules image
   1745 // modules image doesn't have "jre" subdirectory
   1746 size_t len = strlen(buf);
   1747 char* jrebin_p = buf + len;
   1748 jio_snprintf(jrebin_p, buflen-len, "\\jre\\bin\\");
   1749 if (0 != _access(buf, 0)) {
   1750 jio_snprintf(jrebin_p, buflen-len, "\\bin\\");
   1751 }
   1752 len = strlen(buf);
   1753 jio_snprintf(buf + len, buflen-len, "hotspot\\jvm.dll");
   1754 }
   1755 }


Linux, Solaris and Windows all recognize that if they
get to the point where they are appending a "hotspot/..."
path to the jvm_path value, then they are making a fake
path. Why are they making a fake path? Well, the gamma
launcher lets someone use a libjvm.so/jvm.dll that is
not located in the usual place in a JDK. And, you get
to use that libjvm.so/jvm.dll WITH THAT JDK. Sounds
pretty cool and it is pretty cool... until you realize
that there is code in the VM that tries to find other
libraries _relative_ to the libjvm.so/jvm.dll that is
running. Ouch! So enter the above code blocks that fake
a path to the libjvm.so/jvm.dll so that callers to
os::jvm_path() don't get confused.

To make matters more convoluted... yes, I know this is
convoluted enough... The os_bsd.cpp doesn't seem to
realize that it is creating a fake path here. At one
point, the os_bsd.cpp code was a copy of os_linux.cpp
code, but it was obviously tweaked. It will take more
investigation to figure out the right course of action
for os_bsd.cpp