On 12. 02. 19, 13:13, Borislav Petkov wrote: > On Tue, Feb 12, 2019 at 12:51:08PM +0100, Jiri Slaby wrote: >> And what if the LOCAL macros prepend .L automatically? The references >> would need to be via macro or by manually adding .L. I mean: >> >> SYM_CODE_START_LOCAL(function) >> ret >> SYM_CODE_END(function) >> >> And then used as: >> call .Lfunction >> or >> call SYM_LOCAL(function) >> >> Is that too ugly? > > I'd prefer SYM_LOCAL because not everyone is aware of the fact that the > GNU toolchain makes .L-prepended symbols local. The problem with local .L symbols is when debugging: > Local symbols are defined and used within the assembler, but they are > normally not saved in object files. Thus, they are not visible when > debugging. Which means, when I have: > .text > > .globl _start > _start: > call .Lbubak > .type _start STT_FUNC > .size _start, .-_start > > .Lbubak: > movb $0, 0 > .type .Lbubak STT_FUNC > .size .Lbubak, .-.Lbubak and I run it: > (gdb) r > Starting program: /tmp/asm/asm > > Program received signal SIGSEGV, Segmentation fault. > 0x0000000000401006 in ?? () > (gdb) where > #0 0x0000000000401006 in ?? () > #1 0x0000000000401005 in _start () > (gdb) disass > No function contains program counter for selected frame. > (gdb) disass *0x0000000000401006 > No function contains specified address. > (gdb) x/i $pc > => 0x401006: movb $0x0,0x0 > (gdb) x/i 0x0000000000401006 > => 0x401006: movb $0x0,0x0 Which is quite impractical -- disass won't work, only explicit dump via x. And the kernel unwinder would be no more clever. So when patching entry like: > --- a/arch/x86/entry/entry_64.S > +++ b/arch/x86/entry/entry_64.S > @@ -323,6 +323,18 @@ SYM_CODE_START(__switch_to_asm) > jmp __switch_to > SYM_CODE_END(__switch_to_asm) > > +#if 0 > +#define KILLER killer > +#else > +#define KILLER .Lkiller > +#endif > + > +SYM_CODE_START_LOCAL(KILLER) > + UNWIND_HINT_EMPTY > + movb $0, 0 > + ret > +SYM_CODE_END(KILLER) > + > /* > * A newly forked process directly context switches into this address. > * > @@ -332,6 +344,7 @@ SYM_CODE_END(__switch_to_asm) > */ > SYM_CODE_START(ret_from_fork) > UNWIND_HINT_EMPTY > + call KILLER > movq %rax, %rdi > call schedule_tail /* rdi: 'prev' task parameter */ > first results in objtool complaints: > arch/x86/entry/entry_64.o: warning: objtool: .entry.text+0x190: unsupported intra-function call > arch/x86/entry/entry_64.o: warning: objtool: If this is a retpoline, please patch it in with alternatives and annotate it with ANNOTATE_NOSPEC_ALTERNATIVE. and also the crash is misleading: > BUG: unable to handle kernel NULL pointer dereference at 0000000000000000 > #PF error: [WRITE] > PGD 0 P4D 0 > Oops: 0002 [#1] PREEMPT SMP ... > RIP: 0010:__switch_to_asm+0x70/0x80 opposing to classic symbol: > BUG: unable to handle kernel NULL pointer dereference at 0000000000000000 > #PF error: [WRITE] > PGD 0 P4D 0 > Oops: 0002 [#1] PREEMPT SMP ...> RIP: 0010:killer+0x0/0x10 (The killer was appended to the previous function by gas in the former case.) Therefore, I don't think using local .L labels outside of functions is a good idea... regards, -- js suse labs
