> I talked to Andrew about what files he changed in GCC and created and > tested this new patch. Andrew also mentioned changing some assembly > files in glibc but I don't see any use of '.section .note.GNU-stack' in > any assembly files in glibc (for any platform) so I wasn't planning on > creating a glibc to add them to mips glibc assembly language files. > > OK to check in this patch?
I think we need to wait for some conclusion about how the kernel will cope with a non-executable stack before committing this. The discussion is progressing on the glibc and kernel lists. As far as I can tell the kernel will respond to the PT_GNU_STACK segment already for MIPS and then lead to a crash if the FPU emulator is invoked. If this is the case then it would be ideal if we can somehow prevent a stack becoming non-executable unless built against a kernel which is known to be safe for no-execstack. Existing versions of glibc will simply follow suit with the compiler and apply --noexecstack when building assembly files if the compiler emits a .note.GNU-stack section. Given that behaviour then as soon as we enable no-execstack support in the compiler then we will get PT_GNU_STACK on the binaries and risk a crash. I'm struggling to come up with a good way to make this safe on existing kernels. One possibility is that MIPS would have to implement the compiler driven no-execstack support by passing the --no-execstack option to the assembler instead of using the .note.GNU-stack section explicitly. This would avoid triggering the configure check in glibc and allow us to write a MIPS specific check in glibc which looks at both the kernel version and whether the compiler enables no-execstack. Slightly convoluted but safe. I'm not sure how uclibc and musl deal with this issue but we should really check them too (but they are still secondary to glibc I think). Any other ideas? Matthew > > Steve Ellcey > sell...@mips.com > > > > 2014-09-26 Steve Ellcey <sell...@mips.com> > > * config/mips/mips.c (TARGET_ASM_FILE_END): Define. > * libgcc/config/mips/mips16.S: Add .note.GNU-stack section. > * libgcc/config/mips/vr4120-div.S: Ditto. > * libgcc/config/mips/crti.S: Ditto. > * libgcc/config/mips/crtn.S: Ditto. > > > diff --git a/gcc/config/mips/mips.c b/gcc/config/mips/mips.c > index f9713c1..39020d7 100644 > --- a/gcc/config/mips/mips.c > +++ b/gcc/config/mips/mips.c > @@ -19146,6 +19146,9 @@ mips_lra_p (void) > #undef TARGET_LRA_P > #define TARGET_LRA_P mips_lra_p > > +#undef TARGET_ASM_FILE_END > +#define TARGET_ASM_FILE_END file_end_indicate_exec_stack > + > struct gcc_target targetm = TARGET_INITIALIZER; > > #include "gt-mips.h" > diff --git a/libgcc/config/mips/crti.S b/libgcc/config/mips/crti.S > index 6980594..93436c0 100644 > --- a/libgcc/config/mips/crti.S > +++ b/libgcc/config/mips/crti.S > @@ -21,6 +21,10 @@ a copy of the GCC Runtime Library Exception along with > this program; > see the files COPYING3 and COPYING.RUNTIME respectively. If not, see > <http://www.gnu.org/licenses/>. */ > > + > +/* An executable stack is *not* required for these functions. */ > + .section .note.GNU-stack,"",%progbits > + > /* 4 slots for argument spill area. 1 for cpreturn, 1 for stack. > Return spill offset of 40 and 20. Aligned to 16 bytes for n32. */ > > diff --git a/libgcc/config/mips/crtn.S b/libgcc/config/mips/crtn.S > index 0de2d0c..6f2c301 100644 > --- a/libgcc/config/mips/crtn.S > +++ b/libgcc/config/mips/crtn.S > @@ -21,6 +21,9 @@ a copy of the GCC Runtime Library Exception along with > this program; > see the files COPYING3 and COPYING.RUNTIME respectively. If not, see > <http://www.gnu.org/licenses/>. */ > > +/* An executable stack is *not* required for these functions. */ > + .section .note.GNU-stack,"",%progbits > + > /* 4 slots for argument spill area. 1 for cpreturn, 1 for stack. > Return spill offset of 40 and 20. Aligned to 16 bytes for n32. */ > > diff --git a/libgcc/config/mips/mips16.S b/libgcc/config/mips/mips16.S > index dde8939..58e4377 100644 > --- a/libgcc/config/mips/mips16.S > +++ b/libgcc/config/mips/mips16.S > @@ -35,6 +35,10 @@ see the files COPYING3 and COPYING.RUNTIME > respectively. If not, see > values using the soft-float calling convention, but do the actual > operation using the hard floating point instructions. */ > > +/* An executable stack is *not* required for these functions. */ > + .section .note.GNU-stack,"",%progbits > + .previous > + > #if defined _MIPS_SIM && (_MIPS_SIM == _ABIO32 || _MIPS_SIM == _ABIO64) > > /* This file contains 32-bit assembly code. */ > diff --git a/libgcc/config/mips/vr4120-div.S b/libgcc/config/mips/vr4120- > div.S > index 76c4e7a..664d3c3 100644 > --- a/libgcc/config/mips/vr4120-div.S > +++ b/libgcc/config/mips/vr4120-div.S > @@ -26,6 +26,10 @@ see the files COPYING3 and COPYING.RUNTIME > respectively. If not, see > -mfix-vr4120. div and ddiv do not give the correct result when one > of the operands is negative. */ > > +/* An executable stack is *not* required for these functions. */ > + .section .note.GNU-stack,"",%progbits > + .previous > + > .set nomips16 > > #define DIV \ >
