binutils-gdb/ld/testsuite/ld-mips-elf/jal-local-overflow-0.d

#name: MIPS JAL to local symbol overflow 0
#source: jal-local-overflow.s
#as: -EB -32
#ld: -EB -Ttext 0x20000000 -e 0x20000000
#objdump: -dr --prefix-addresses --show-raw-insn
#dump: jal-global-overflow-0.d
MIPS/BFD: Enable local R_MIPS_26 overflow detection The original MIPS SVR4 psABI defines the calculation for the R_MIPS_26 relocation in a complex way, as follows[1]: Name Value Field Symbol Calculation R_MIPS_26 4 T-targ26 local (((A << 2) \| \ (P & 0xf0000000)) + S) >> 2 4 T-targ26 external (sign-extend(A << 2) + S) >> 2 This is further clarified, by correcting typos (already applied in the excerpt above) in the 64-bit psABI extension[2]. A note is included in both documents to specify that for the purpose of relocation processing a local symbol is one with binding STB_LOCAL and type STT_SECTION, and otherwise, a symbol is external. We have both calculations implemented for the R_MIPS_26 relocation, and by extension also for the R_MIPS16_26 and R_MICROMIPS_26_S1 relocations, from now on collectively called jump relocations. However our code uses a different condition to tell local and external symbols apart, that is it only checks for the STB_LOCAL binding and ignores the symbol type, however for REL relocations only. The external calculation is used for all RELA jump relocations. In reality the difference matters for jump relocations referring local MIPS16 and, as from recent commit 44d3da233815 ("MIPS/GAS: Treat local jump relocs the same no matter if REL or RELA"), also local microMIPS symbols. Such relocations are not converted to refer to corresponding section symbols instead and retain the original local symbol reference. It can be inferred from the relocation calculation definitions that the addend is effectively unsigned for the local case and explicitly signed for the external case. With the REL relocation format it makes sense given the limited range provided for by the field being relocated: the use of an unsigned addend expands the range by one bit for the local case, because a negative offset from a section symbol makes no sense, and any usable negative offset from the original local symbol will have worked out positive if converted to a section-relative reference. In the external case a signed addend gives more flexibility as offsets both negative and positive can be used with a symbol. Any such offsets will typically have a small value. The inclusion of the (P & 0xf0000000) component, ORed in the calculation in the local case, seems questionable as bits 31:28 are not included in the relocatable field and are masked out as the relocation is applied. Their value is therefore irrelevant for output processing, the relocated field ends up the same regardless of their value. They could be used for overflow detection, however this is precluded by adding them to bits 31:28 of the symbol referred, as the sum will not correspond to the value calculated by the processor at run time whenever bits 31:28 of the symbol referred are not all zeros, even though it is valid as long they are the same as bits 31:28 of P. We deal with this problem by ignoring any overflow resulting from the local calculation. This however makes us miss genuine overflow cases, where 31:28 of the symbol referred are different from bits 31:28 of P, and non-functional code is produced. Given the situation, for the purpose of overflow detection we can change our code to follow the original psABI and only treat the in-place addend as unsigned in the section symbol case, permitting jumps to offsets 128MiB and above into section. Sections so large may be uncommon, but still a reasonable use case. On the other hand such large offsets from regular local symbols are not expected and it makes sense to support (possibly small) negative offsets instead, also in consistency with what we do for global symbols. Drop the (P & 0xf0000000) component then, treat the addend as signed with local non-section symbols and also detect an overflow in the result of such calculation with local symbols. NB it does not affect the value computed for the relocatable field, it only affects overflow detection. References: [1] "SYSTEM V APPLICATION BINARY INTERFACE, MIPS RISC Processor Supplement, 3rd Edition", Figure 4-11: "Relocation Types", p. 4-19 <http://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf> [2] "64-bit ELF Object File Specification, Draft Version 2.5", Table 32 "Relocation Types", p. 45 <http://techpubs.sgi.com/library/manuals/4000/007-4658-001/pdf/007-4658-001.pdf> bfd/ * elfxx-mips.c (mips_elf_calculate_relocation): <R_MIPS16_26> <R_MIPS_26, R_MICROMIPS_26_S1>: Drop the region bits of the reloc location from calculation, treat the addend as signed with local non-section symbols and enable overflow detection. ld/ * testsuite/ld-mips-elf/jal-global-overflow-0.d: New test. * testsuite/ld-mips-elf/jal-global-overflow-1.d: New test. * testsuite/ld-mips-elf/jal-local-overflow-0.d: New test. * testsuite/ld-mips-elf/jal-local-overflow-1.d: New test. * testsuite/ld-mips-elf/jal-global-overflow.s: New test source. * testsuite/ld-mips-elf/jal-local-overflow.s: New test source. * testsuite/ld-mips-elf/mips-elf.exp: Run the new tests. 2016-05-28 17:30:22 +08:00			`#name: MIPS JAL to local symbol overflow 0`
			`#source: jal-local-overflow.s`
			`#as: -EB -32`
			`#ld: -EB -Ttext 0x20000000 -e 0x20000000`
			`#objdump: -dr --prefix-addresses --show-raw-insn`
			`#dump: jal-global-overflow-0.d`