RFR: 8313396: Portable implementation of FORBID_C_FUNCTION and ALLOW_C_FUNCTION

[Julian Waters]

On Sun, 29 Dec 2024 08:11:07 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review this change to how HotSpot prevents the use of certain C library
> functions (e.g. poisons references to those functions), while permitting a
> subset to be used in restricted circumstances.  Reasons for poisoning a
> function include it being considered obsolete, or a security concern, or there
> is a HotSpot function (typically in the os:: namespace) providing similar
> functionality that should be used instead.
> 
> The old mechanism, based on -Wattribute-warning and the associated attribute,
> only worked for gcc.  (Clang's implementation differs in an important way from
> gcc, which is the subject of a clang bug that has been open for years.  MSVC
> doesn't provide a similar mechanism.)  It also had problems with LTO, due to a
> gcc bug.
> 
> The new mechanism is based on deprecation warnings, using [[deprecated]]
> attributes. We redeclare or forward declare the functions we want to prevent
> use of as being deprecated.  This relies on deprecation warnings being
> enabled, which they already are in our build configuration.  All of our
> supported compilers support the [[deprecated]] attribute.
> 
> Another benefit of using deprecation warnings rather than warning attributes
> is the time when the check is performed.  Warning attributes are checked only
> if the function is referenced after all optimizations have been performed.
> Deprecation is checked during initial semantic analysis.  That's better for
> our purposes here.  (This is also part of why gcc LTO has problems with the
> old mechanism, but not the new.)
> 
> Adding these redeclarations or forward declarations isn't as simple as
> expected, due to differences between the various compilers.  We hide the
> differences behind a set of macros, FORBID_C_FUNCTION and related macros.  See
> the compiler-specific parts of those macros for details.
> 
> In some situations we need to allow references to these poisoned functions.
> 
> One common case is where our poisoning is visible to some 3rd party code we
> don't want to modify.  This is typically 3rd party headers included in HotSpot
> code, such as from Google Test or the C++ Standard Library.  For these the
> BEGIN/END_ALLOW_FORBIDDEN_FUNCTIONS pair of macros are used demark the context
> where such references are permitted.
> 
> Some of the poisoned functions are needed to implement associated HotSpot os::
> functions, or in other similarly restricted contexts.  For these, a wrapper
> function is provided that calls the poisoned function with the warning
> suppressed. These wrappers are defined in the permit_fo...

src/hotspot/os/windows/permitForbiddenFunctions_windows.hpp line 41:

> 39:   return ::_fullpath(absPath, relPath, maxLength);
> 40: }
> 41: 

Could we forbid the non Standard _snprintf too?

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PR Review Comment: https://git.openjdk.org/jdk/pull/22890#discussion_r1899086776