Question about alignment of struct _Unwind_Exception

[Matheus Castanho]

Hi,

Recently I've been investigating an issue with an invalid TLS access on
powerpc64le when __libc_malloc tries to read SINGLE_THREAD_P [1]. The
issue could only be reproduced when using a glibc compiled with
-mcpu=power10. We found out that the size of 'struct pthread' was
changing between compiling with -mcpu=power9 and -mcpu=power10.

After inspecting that struct we found out that one specific field was
changing sizes:
	struct _Unwind_Exception exc;

That struct is declared without a fixed alignment value:

struct _Unwind_Exception
{
  _Unwind_Exception_Class exception_class;
  _Unwind_Exception_Cleanup_Fn exception_cleanup;
  _Unwind_Word private_1;
  _Unwind_Word private_2;
  /* @@@ The IA-64 ABI says that this structure must be double-word aligned.
     Taking that literally does not make much sense generically.  Instead we
     provide the maximum alignment required by any type for the machine.  */
} __attribute__((__aligned__));

The GCC manual says [2]:

> Specifying no alignment argument implies the maximum alignment for the
target

The maximum alignment has been recently increased for P10, which we
suspect is causing the behavior we see.

Is there an specific reason why a fixed alignment value was not used for
struct _Unwind_Exception?

[1] https://github.com/advancetoolchain/advance-toolchain/issues/1780
[2]
https://gcc.gnu.org/onlinedocs/gcc-10.1.0/gcc/Common-Variable-Attributes.html#Common-Variable-Attributes

Thanks,
Matheus Castanho