On Tue, May 24, 2016 at 09:46:58AM +0200, Francesco Zappa Nardelli wrote: > Dear David and all > > If you could produce a small object file.. > > > Invoking readelf on /lib/x86_64-linux-gnu/libc.so.6 is enough to observe > this (I am on Ubuntu 14.04.3 LTS (GNU/Linux 3.13.0-68-generic x86_64)): > > $ readelf -wf /lib/x86_64-linux-gnu/libc.so.6 (and search for S in the > augmentation string)
The only zRS I see in my libc.so.6 is the __restore_rt trampoline in sigaction.c. https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/x86_64/sigaction.c;h=71ac05c4bc01c560935f3bbd4306e3aeacb6d9be;hb=HEAD The CIE doesn't contain any (non-nop) instructions, so guess readelf -wF's default for no instructions at all is the first register. Does it really matter? The (only) FDE that refers to this CIE defines the CFA expression as the very first thing in it, so it isn't really ever used. Though, perhaps you might want to file a request to move the do_cfa_expr macro into the CIE from the FDE - I think DW_CFA_def_cfa_expression (DW_OP_breg7 (rsp): 160; DW_OP_deref) is 6 bytes and therefore would fit exactly into the padding at the end of the CIE. > 00002690 0000000000000014 00000000 CIE > Version: 1 > Augmentation: "zRS" > Code alignment factor: 1 > Data alignment factor: -8 > Return address column: 16 > Augmentation data: 1b > > DW_CFA_nop > DW_CFA_nop > DW_CFA_nop > DW_CFA_nop > DW_CFA_nop > DW_CFA_nop > > while readelf -wF gives: > > 00002690 0000000000000014 00000000 CIE "zRS" cf=1 df=-8 ra=16 > LOC CFA > 0000000000000000 rax+0 > > Remark: objdump emits the same output. > > I believe that this is the unwinding information for the longjmp call, and > the unwinding infos are computed from a structure on the stack representing > the processor state to which we want longjmp: > > 000026a8 000000000000007c 0000001c FDE cie=00002690 > pc=0000000000036d3f..0000000000036d49 > LOC CFA > rax rdx rcx rbx rsi rdi rbp rsp r8 r9 > r10 r11 r12 r13 r14 r15 ra > 0000000000036d3f exp > exp exp exp exp exp exp exp exp exp exp exp > exp exp exp exp exp > exp > > 000026a8 000000000000007c 0000001c FDE cie=00002690 > pc=0000000000036d3f..0000000000036d49 > DW_CFA_def_cfa_expression (DW_OP_breg7 (rsp): 160; DW_OP_deref) > DW_CFA_expression: r8 (r8) (DW_OP_breg7 (rsp): 40) > DW_CFA_expression: r9 (r9) (DW_OP_breg7 (rsp): 48) > … (and so on for all registers)… > > So it makes sense to ignore whatever in the CIE here. > > I am just surprised that readelf/objdump prints “rax+0”: I would have > expected “u” as the CFA in the CIE is never computed by the CIE bytecode. > Wonder if I am missing something... Jakub _______________________________________________ Dwarf-Discuss mailing list Dwarf-Discuss@lists.dwarfstd.org http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org
