Re: [PATCH v6 02/27] x86: Use symbol name in jump table for PIE support
On Thu, Feb 7, 2019 at 9:11 AM Borislav Petkov wrote: > > On Thu, Feb 07, 2019 at 09:04:45AM -0800, Thomas Garnier wrote: > > I assume that's an optimisation done by gcc later. > > So why is that change even needed? Where does it break? > > > The P modifier in the documentation does state that it is used to > > generate PIC code. > > The documentation says: > > "If used for a function, print the PLT suffix and generate PIC code. For > example, emit foo@PLT instead of ’foo’ for the function foo()." > > when you use %P for a function. Which is not how it is used here. I did more checks about that. I think Ard's patch to make jump label relative actually fixed the issue I had with them. Thanks for spotting this, I will do additional checks and look at removing this change. > > -- > Regards/Gruss, > Boris. > > Good mailing practices for 400: avoid top-posting and trim the reply.
Re: [PATCH v6 02/27] x86: Use symbol name in jump table for PIE support
On Thu, Feb 07, 2019 at 09:04:45AM -0800, Thomas Garnier wrote: > I assume that's an optimisation done by gcc later. So why is that change even needed? Where does it break? > The P modifier in the documentation does state that it is used to > generate PIC code. The documentation says: "If used for a function, print the PLT suffix and generate PIC code. For example, emit foo@PLT instead of ’foo’ for the function foo()." when you use %P for a function. Which is not how it is used here. -- Regards/Gruss, Boris. Good mailing practices for 400: avoid top-posting and trim the reply.
Re: [PATCH v6 02/27] x86: Use symbol name in jump table for PIE support
On Thu, Feb 7, 2019 at 4:17 AM Borislav Petkov wrote: > > On Thu, Jan 31, 2019 at 11:24:09AM -0800, Thomas Garnier wrote: > > Replace the %c constraint with %P. The %c is incompatible with PIE > > because it implies an immediate value whereas %P reference a symbol. > > How so? > > AFAIK, %c requires a constant operand and if %P is used to print a > constant, it simply drops syntax-specific prefixes and does a bare > constant. > > I guess that here > > https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#x86Operandmodifiers > > is not entirely correct as it should not say "If used for a constant" > for %P but say "symbol or constant". > > But before/after asm doesn't show any difference. So what gives? I assume that's an optimisation done by gcc later. The P modifier in the documentation does state that it is used to generate PIC code. > > before: > # 39 "./arch/x86/include/asm/jump_label.h" 1 > 1: > .byte 0xe9 > .long .L241 - 2f # > 2: > .pushsection __jump_table, "aw" > .balign 8 > .long 1b - ., .L241 - . # > .quad __use_tsc + 1 - .#, > .popsection > > after: > # 39 "./arch/x86/include/asm/jump_label.h" 1 > 1: > .byte 0xe9 > .long .L241 - 2f # > 2: > .pushsection __jump_table, "aw" > .balign 8 > .long 1b - ., .L241 - . # > .quad __use_tsc+1 - . # > .popsection > > -- > Regards/Gruss, > Boris. > > Good mailing practices for 400: avoid top-posting and trim the reply.
Re: [PATCH v6 02/27] x86: Use symbol name in jump table for PIE support
On Thu, Jan 31, 2019 at 11:24:09AM -0800, Thomas Garnier wrote: > Replace the %c constraint with %P. The %c is incompatible with PIE > because it implies an immediate value whereas %P reference a symbol. How so? AFAIK, %c requires a constant operand and if %P is used to print a constant, it simply drops syntax-specific prefixes and does a bare constant. I guess that here https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#x86Operandmodifiers is not entirely correct as it should not say "If used for a constant" for %P but say "symbol or constant". But before/after asm doesn't show any difference. So what gives? before: # 39 "./arch/x86/include/asm/jump_label.h" 1 1: .byte 0xe9 .long .L241 - 2f # 2: .pushsection __jump_table, "aw" .balign 8 .long 1b - ., .L241 - . # .quad __use_tsc + 1 - .#, .popsection after: # 39 "./arch/x86/include/asm/jump_label.h" 1 1: .byte 0xe9 .long .L241 - 2f # 2: .pushsection __jump_table, "aw" .balign 8 .long 1b - ., .L241 - . # .quad __use_tsc+1 - . # .popsection -- Regards/Gruss, Boris. Good mailing practices for 400: avoid top-posting and trim the reply.
