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libcxxrt: add padding in __cxa_allocate_* to fix alignment

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The addition of the referenceCount to __cxa_allocate_exception put the
unwindHeader at offset 0x58 in __cxa_exception, but it requires 16-byte
alignment. In order to avoid changing the current __cxa_exception ABI
(and thus breaking its consumers), add explicit padding in the
allocation routines (and account for it when freeing).

This is intended as a lower-risk change for FreeBSD 11. A "more correct"
fix should be prepared for upstream and -CURRENT.

Reviewed by:	dim
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D7271
This commit is contained in:
Ed Maste 2016-07-21 18:20:35 +00:00
parent bb04182c90
commit 3c4fd2463b
1 changed files with 24 additions and 6 deletions
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30
contrib/libcxxrt/exception.cc
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@ -572,6 +572,19 @@ static void free_exception(char *e)
} }
} }
#ifdef __LP64__
/**
* There's an ABI bug in __cxa_exception: unwindHeader requires 16-byte
* alignment but it was broken by the addition of the referenceCount.
* The unwindHeader is at offset 0x58 in __cxa_exception. In order to keep
* compatibility with consumers of the broken __cxa_exception, explicitly add
* padding on allocation (and account for it on free).
*/
static const int exception_alignment_padding = 8;
#else
static const int exception_alignment_padding = 0;
#endif
/** /**
* Allocates an exception structure. Returns a pointer to the space that can * Allocates an exception structure. Returns a pointer to the space that can
* be used to store an object of thrown_size bytes. This function will use an * be used to store an object of thrown_size bytes. This function will use an
@ -580,16 +593,19 @@ static void free_exception(char *e)
*/ */
extern "C" void *__cxa_allocate_exception(size_t thrown_size) extern "C" void *__cxa_allocate_exception(size_t thrown_size)
{ {
size_t size = thrown_size + sizeof(__cxa_exception); size_t size = exception_alignment_padding + sizeof(__cxa_exception) +
thrown_size;
char *buffer = alloc_or_die(size); char *buffer = alloc_or_die(size);
return buffer+sizeof(__cxa_exception); return buffer + exception_alignment_padding + sizeof(__cxa_exception);
} }
extern "C" void *__cxa_allocate_dependent_exception(void) extern "C" void *__cxa_allocate_dependent_exception(void)
{ {
size_t size = sizeof(__cxa_dependent_exception); size_t size = exception_alignment_padding +
sizeof(__cxa_dependent_exception);
char *buffer = alloc_or_die(size); char *buffer = alloc_or_die(size);
return buffer+sizeof(__cxa_dependent_exception); return buffer + exception_alignment_padding +
sizeof(__cxa_dependent_exception);
} }
/** /**
@ -617,7 +633,8 @@ extern "C" void __cxa_free_exception(void *thrown_exception)
} }
} }
free_exception(reinterpret_cast<char*>(ex)); free_exception(reinterpret_cast<char*>(ex) -
exception_alignment_padding);
} }
static void releaseException(__cxa_exception *exception) static void releaseException(__cxa_exception *exception)
@ -644,7 +661,8 @@ void __cxa_free_dependent_exception(void *thrown_exception)
{ {
releaseException(realExceptionFromException(reinterpret_cast<__cxa_exception*>(ex))); releaseException(realExceptionFromException(reinterpret_cast<__cxa_exception*>(ex)));
} }
free_exception(reinterpret_cast<char*>(ex)); free_exception(reinterpret_cast<char*>(ex) -
exception_alignment_padding);
} }
/** /**