8264285: Do not support FLAG_SET_XXX for VM flags of string type

[iklam]

We have two versions of JVMFlagAccess::ccstrAtPut() that are slightly different.

The following version is supposed to be used only by the FLAG_SET_{CMDLINE,ERGO,MGMT} macros. However, it's not used anywhere in the HotSpot source code:

JVMFlag::Error JVMFlagAccess::ccstrAtPut(JVMFlagsEnum flag, ccstr value, JVMFlagOrigin origin) {
  JVMFlag* faddr = JVMFlag::flag_from_enum(flag);
  assert(faddr->is_ccstr(), "wrong flag type");
  ccstr old_value = faddr->get_ccstr();
  trace_flag_changed<ccstr, EventStringFlagChanged>(faddr, old_value, value, origin);
  char* new_value = os::strdup_check_oom(value);
  faddr->set_ccstr(new_value);
  if (!faddr->is_default() && old_value != NULL) {
    // Prior value is heap allocated so free it.
    FREE_C_HEAP_ARRAY(char, old_value);
  }
  faddr->set_origin(origin);
  return JVMFlag::SUCCESS;
}

It's not clear whether this unused version is actually correct since the last JVMFlag rewrite in JDK-8081833, due to complete lack of testing. Let's remove this version to simplify code maintenance.

If you need to modify flags of the string type, do not use FLAG_SET_{CMDLINE,ERGO,MGMT}. (A static_assert is added to prevent this). Instead, use the remaining version of JVMFlagAccess::ccstrAtPut().

---

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Issue

• JDK-8264285: Do not support FLAG_SET_XXX for VM flags of string type

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$ git checkout pull/3219

To update a local copy of the PR:
$ git checkout pull/3219
$ git pull https://git.openjdk.java.net/jdk pull/3219/head

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/label add hotspot-dev

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[mlbridge]

Webrevs

• 00: Full (4fe5df9)

[mlbridge]

Mailing list message from David Holmes on hotspot-dev:

Hi Ioi,

On 27/03/2021 3:01 am, Ioi Lam wrote:

We have two versions of `JVMFlagAccess::ccstrAtPut()` that are slightly different.

The following version is supposed to be used only by the `FLAG_SET_{CMDLINE,ERGO,MGMT}` macros. However, it's not used anywhere in the HotSpot source code:

JVMFlag::Error JVMFlagAccess::ccstrAtPut(JVMFlagsEnum flag, ccstr value, JVMFlagOrigin origin) {
JVMFlag* faddr = JVMFlag::flag_from_enum(flag);
assert(faddr->is_ccstr(), "wrong flag type");
ccstr old_value = faddr->get_ccstr();
trace_flag_changed<ccstr, EventStringFlagChanged>(faddr, old_value, value, origin);
char* new_value = os::strdup_check_oom(value);
faddr->set_ccstr(new_value);
if (!faddr->is_default() && old_value != NULL) {
// Prior value is heap allocated so free it.
FREE_C_HEAP_ARRAY(char, old_value);
}
faddr->set_origin(origin);
return JVMFlag::SUCCESS;
}

It's not clear whether this unused version is actually correct since the last JVMFlag rewrite in [JDK-8081833](https://bugs.openjdk.java.net/browse/JDK-8081833), due to complete lack of testing. Let's remove this version to simplify code maintenance.

I agree it may not work because it is untested but I don't agree it
should be removed as we may want to set a string flag this way ...

If you need to modify flags of the string type, do not use `FLAG_SET_{CMDLINE,ERGO,MGMT}`. (A `static_assert` is added to prevent this). Instead, use the remaining version of `JVMFlagAccess::ccstrAtPut()`.

... and using ccstrAtPut doesn't update the origin of the flag as might
be desired when using the macros.

I'd rather see a test introduced to sanity check the operation if possible.

There is a tension between writing balanced API's and not introducing
"dead code". We seem to be swinging the style pendulum more to the "get
rid of all dead code" end, rather than considering the utility of
writing a balanced API that makes a subsystem functionally complete. :(

David
-----

[iklam]

If you need to modify flags of the string type, do not use FLAG_SET_{CMDLINE,ERGO,MGMT}. (A static_assert is added to prevent this). Instead, use the remaining version of JVMFlagAccess::ccstrAtPut().

... and using ccstrAtPut doesn't update the origin of the flag as might
be desired when using the macros.

This is the version I removed:

  static JVMFlag::Error ccstrAtPut(JVMFlagsEnum flag, ccstr value, JVMFlagOrigin origin);

This is the remaining version:

  static JVMFlag::Error ccstrAtPut(JVMFlag* flag, ccstr* value, JVMFlagOrigin origin);

So they are practically the same API. The origin is changed in both. The only difference is they have unobvious subtle difference in how they handle the buffer allocation.

[mlbridge]

Mailing list message from David Holmes on hotspot-dev:

On 29/03/2021 3:53 pm, Ioi Lam wrote:

On Fri, 26 Mar 2021 16:21:03 GMT, Ioi Lam <iklam at openjdk.org> wrote:

We have two versions of `JVMFlagAccess::ccstrAtPut()` that are slightly different.

The following version is supposed to be used only by the `FLAG_SET_{CMDLINE,ERGO,MGMT}` macros. However, it's not used anywhere in the HotSpot source code:

JVMFlag::Error JVMFlagAccess::ccstrAtPut(JVMFlagsEnum flag, ccstr value, JVMFlagOrigin origin) {
JVMFlag* faddr = JVMFlag::flag_from_enum(flag);
assert(faddr->is_ccstr(), "wrong flag type");
ccstr old_value = faddr->get_ccstr();
trace_flag_changed<ccstr, EventStringFlagChanged>(faddr, old_value, value, origin);
char* new_value = os::strdup_check_oom(value);
faddr->set_ccstr(new_value);
if (!faddr->is_default() && old_value != NULL) {
// Prior value is heap allocated so free it.
FREE_C_HEAP_ARRAY(char, old_value);
}
faddr->set_origin(origin);
return JVMFlag::SUCCESS;
}

It's not clear whether this unused version is actually correct since the last JVMFlag rewrite in [JDK-8081833](https://bugs.openjdk.java.net/browse/JDK-8081833), due to complete lack of testing. Let's remove this version to simplify code maintenance.

If you need to modify flags of the string type, do not use `FLAG_SET_{CMDLINE,ERGO,MGMT}`. (A `static_assert` is added to prevent this). Instead, use the remaining version of `JVMFlagAccess::ccstrAtPut()`.

... and using ccstrAtPut doesn't update the origin of the flag as might
be desired when using the macros.

This is the version I removed:

static JVMFlag::Error ccstrAtPut(JVMFlagsEnum flag, ccstr value, JVMFlagOrigin origin);

This is the remaining version:

static JVMFlag::Error ccstrAtPut(JVMFlag* flag, ccstr* value, JVMFlagOrigin origin);

So they are practically the same API. The origin is changed in both. The only difference is they have unobvious subtle difference in how they handle the buffer allocation.

Sorry I'm confused, if both support setting the origin why can't you use
this version with the macros?

Thanks,
David
-----

[iklam]

Thanks to @dholmes-ora for pointing out the problems with this PR. I have changed the REF to fix the problem differently. To avoid confusion, I am closing this PR.

I opened a new PR (#3254) for the new fix.