On 06/24/2013 12:33 PM, Jakub Jelinek wrote: > On Mon, Jun 24, 2013 at 12:06:27PM +0530, Vineet Gupta wrote: >> I had a question about interpretation of FDE's CIE_pointer field (for >> .debug_frame) >> >> The spec say (from dwarf4 version although it really doesn't matter): >> >> "2. CIE_pointer (4 or 8 bytes, see Section 7.4) >> A constant offset into the .debug_frame section that denotes the CIE that is >> associated with this FDE." >> >> Does "offset" above mean offset from current location (in FDE) to CIE or >> does it >> mean offset from start of .debug_frame to the CIE. Per Ian Lance Taylor's >> blog, >> and if I'm interpreting it correctly, >> (http://www.airs.com/blog/archives/460) it >> seems to be latter. > CIE_pointer in .debug_frame is relative to the start of the .debug_frame > section. In .eh_frame section it is encoded based on the selected encoding, > often relative to the start of the CIE_pointer. > >> ... >> .LSFDE0: >> .4byte @.LEFDE0-@.LASFDE0 --> FDE >> .LASFDE0: >> .4byte @.Lframe0 --> CIE pointer - direct reference to CI >> (not >> offset from start of .debug_frame) > This looks fine.
Pardon me if I sound dense (not really my area of expertise), wowever, the 2nd word in FDE above (@.Lframe0) is a direct reference to start of .debbug_frame shouldn't it be something like @.Lframe0 - @.Lframe0 i.e. zero. Or do you think asseblmer/linker need to "interpret it" like that. >> .... >> >> This direct reference to start of CIE is causing objdump to reference >> invalid CIE >> and hence print invalid CFI - although the CFI itself is valid since the >> code_factor and such are seeded from a bogus CIE. >> >> 00000060 00000014 80e0c000 FDE cie=48b25ff8 pc=80a680d4..80a6810a >> ... ^^^^^^ > The 48b25ff8 looks wrong though, it would really surprise me if .debug_frame > section was more than 1GB big. So perhaps your assembler or linker don't > handle it properly? Exactly, although this is Linux kernel image which is linked at start of untranslated address space i.e. 0x8000_0000 onwards. The point however is that cie value above should read zero - not 0x8abcdefg since it is relative to start of .debug_frame -Vineet
