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GZJH5XjSpHs3YaKVwslvTSLVI__Qb8sl X-Proofpoint-ORIG-GUID: GZJH5XjSpHs3YaKVwslvTSLVI__Qb8sl Precedence: bulk List-ID: X-Mailing-List: linux-toolchains@vger.kernel.org Hi, During one of our discussions at LSF/MM/BPF, there was a confusion around what value is stored in the "Start PC" in the SFrame Function Descriptor Entry (FDE). Short answer: The start PC of function, as encoded in the SFrame FDE is a 4-byte non-zero signed value representing the offset to the start instruction of the function, from the start of the SFrame section. Longer version: (Excerpts below use x86_64 artifacts) For a toy program with 4 functions, there are 4 FDEs (See below, 1 reloc per FDE). Each reloc is for the respective function symbol in the object file. $ readelf --relocs deep_hello_sframe.o Relocation section '.rela.sframe' at offset 0x4e0 contains 4 entries: Offset Info Type Sym. Value Sym. Name + Addend 00000000001c 000200000002 R_X86_64_PC32 0000000000000000 .text + 0 00000000002d 000200000002 R_X86_64_PC32 0000000000000000 .text + 19 00000000003e 000200000002 R_X86_64_PC32 0000000000000000 .text + 49 00000000004f 000200000002 R_X86_64_PC32 0000000000000000 .text + 79 As you see below, the "pointed to" entity is the function start location. These R_X86_64_PC32 relocations in SFrame FDEs are resolved at link-time. Symbol table '.symtab' contains XX entries: Num: Value Size Type Bind Vis Ndx Name ... 4: 0000000000000000 25 FUNC GLOBAL DEFAULT 1 baz ... 6: 0000000000000019 48 FUNC GLOBAL DEFAULT 1 bar 7: 0000000000000049 48 FUNC GLOBAL DEFAULT 1 foo 8: 0000000000000079 51 FUNC GLOBAL DEFAULT 1 main ... Now when linking this object, first, the linker resolves these the standard R_X86_64_PC32 relocations generically as per the ABI specification: Calculation = S + A - P. In the above, S = value of the .text section symbol A = Addend, value will be as shown in the "Sym. Name + Addend" column P = Represents the place (section offset or address) of the storage unit being relocated Now the P here will be value of the .sframe section symbol + Offset as shown in the "Offset" column, i.e., sframe_vma + Offset So the value in the first step is: (1) Value = S + A - (sframe_vma + Offset) Next, when merging the .sframe section, the linker does an additional step of making this value relative to the start of the SFrame section by doing: (2) Value += Offset So, at the end of the above two steps, the value is: (3) Value = S + A - sframe_vma This (3) is the value stored in "Start PC of function" in the SFrame FDE. I will clarify the SPEC to reflect that the start PC of the function, as encoded in the FDE is a 4-byte non-zero signed value representing the offset of the start PC relative to the start of the SFrame section. This means that given a PC, the stack tracer will need to lookup the SFrame FDE with lookup_address = PC - sframe_vma. Hope this helps.