From e2212738d40877a8a54e5728999acb7f035f66f3 Mon Sep 17 00:00:00 2001 From: Thomas Preud'homme Date: Sun, 4 Nov 2012 00:40:05 +0100 Subject: [PATCH] Generate PLT thumb stub only when necessary Generate PLT thumb stub for an ARM PLT entry only when at least one Thumb instruction branches to that entry. This is a rewrite of the previous patch. --- tcc.h | 13 ++++++++++--- tccelf.c | 66 ++++++++++++++++++++++++++++++++++++++++------------------------ 2 files changed, 51 insertions(+), 28 deletions(-) diff --git a/tcc.h b/tcc.h index 4e88782c..9e7c574b 100644 --- a/tcc.h +++ b/tcc.h @@ -492,6 +492,13 @@ typedef struct ASMOperand { } ASMOperand; #endif +struct sym_attr { + unsigned long got_offset; +#ifdef TCC_TARGET_ARM + unsigned char plt_thumb_stub:1; +#endif +}; + struct TCCState { unsigned output_type : 8; unsigned reloc_output : 1; @@ -524,11 +531,11 @@ struct TCCState { Section **priv_sections; int nb_priv_sections; /* number of private sections */ - /* got handling */ + /* got & plt handling */ Section *got; Section *plt; - unsigned long *got_offsets; - int nb_got_offsets; + struct sym_attr *sym_attrs; + int nb_sym_attrs; /* give the correspondance from symtab indexes to dynsym indexes */ int *symtab_to_dynsym; diff --git a/tccelf.c b/tccelf.c index c5690dd4..da81d035 100644 --- a/tccelf.c +++ b/tccelf.c @@ -581,7 +581,7 @@ ST_FUNC void relocate_section(TCCState *s1, Section *s) break; case R_386_GOT32: /* we load the got offset */ - *(int *)ptr += s1->got_offsets[sym_index]; + *(int *)ptr += s1->sym_attrs[sym_index].got_offset; break; case R_386_16: if (s1->output_format != TCC_OUTPUT_FORMAT_BINARY) { @@ -760,7 +760,7 @@ ST_FUNC void relocate_section(TCCState *s1, Section *s) break; case R_ARM_GOT_BREL: /* we load the got offset */ - *(int *)ptr += s1->got_offsets[sym_index]; + *(int *)ptr += s1->sym_attrs[sym_index].got_offset; break; case R_ARM_COPY: break; @@ -866,14 +866,14 @@ ST_FUNC void relocate_section(TCCState *s1, Section *s) } #endif *(int *)ptr += (s1->got->sh_addr - addr + - s1->got_offsets[sym_index] - 4); + s1->sym_attrs[sym_index].got_offset - 4); break; case R_X86_64_GOTTPOFF: *(int *)ptr += val - s1->got->sh_addr; break; case R_X86_64_GOT32: /* we load the got offset */ - *(int *)ptr += s1->got_offsets[sym_index]; + *(int *)ptr += s1->sym_attrs[sym_index].got_offset; break; #else #error unsupported processor @@ -943,23 +943,23 @@ static int prepare_dynamic_rel(TCCState *s1, Section *sr) return count; } -static void put_got_offset(TCCState *s1, int index, unsigned long val) +static struct sym_attr *alloc_sym_attr(TCCState *s1, int index) { int n; - unsigned long *tab; + struct sym_attr *tab; - if (index >= s1->nb_got_offsets) { + if (index >= s1->nb_sym_attrs) { /* find immediately bigger power of 2 and reallocate array */ n = 1; while (index >= n) n *= 2; - tab = tcc_realloc(s1->got_offsets, n * sizeof(unsigned long)); - s1->got_offsets = tab; - memset(s1->got_offsets + s1->nb_got_offsets, 0, - (n - s1->nb_got_offsets) * sizeof(unsigned long)); - s1->nb_got_offsets = n; + tab = tcc_realloc(s1->sym_attrs, n * sizeof(*s1->sym_attrs)); + s1->sym_attrs = tab; + memset(s1->sym_attrs + s1->nb_sym_attrs, 0, + (n - s1->nb_sym_attrs) * sizeof(*s1->sym_attrs)); + s1->nb_sym_attrs = n; } - s1->got_offsets[index] = val; + return &s1->sym_attrs[index]; } /* XXX: suppress that */ @@ -1023,11 +1023,11 @@ static void put_got_entry(TCCState *s1, build_got(s1); /* if a got entry already exists for that symbol, no need to add one */ - if (sym_index < s1->nb_got_offsets && - s1->got_offsets[sym_index] != 0) + if (sym_index < s1->nb_sym_attrs && + s1->sym_attrs[sym_index].got_offset) return; - - put_got_offset(s1, sym_index, s1->got->data_offset); + + alloc_sym_attr(s1, sym_index)->got_offset = s1->got->data_offset; if (s1->dynsym) { sym = &((ElfW(Sym) *)symtab_section->data)[sym_index]; @@ -1104,10 +1104,13 @@ static void put_got_entry(TCCState *s1, put32(p + 12, 0xe5bef008); } - p = section_ptr_add(plt, 20); - put32(p , 0x4778); // bx pc - put32(p+2, 0x46c0); // nop - p += 4; + if (s1->sym_attrs[sym_index].plt_thumb_stub) { + p = section_ptr_add(plt, 20); + put32(p , 0x4778); // bx pc + put32(p+2, 0x46c0); // nop + p += 4; + } else + p = section_ptr_add(plt, 16); put32(p , 0xe59fc004); // ldr ip, [pc, #4] // offset in GOT put32(p+4, 0xe08fc00c); // add ip, pc, ip // absolute address or offset put32(p+8, 0xe59cf000); // ldr pc, [ip] // load absolute address or load offset @@ -1494,9 +1497,9 @@ ST_FUNC void fill_got_entry(TCCState *s1, ElfW_Rel *rel) ElfW(Sym) *sym = &((ElfW(Sym) *) symtab_section->data)[sym_index]; unsigned long offset; - if (sym_index >= s1->nb_got_offsets) + if (sym_index >= s1->nb_sym_attrs) return; - offset = s1->got_offsets[sym_index]; + offset = s1->sym_attrs[sym_index].got_offset; section_reserve(s1->got, offset + PTR_SIZE); #ifdef TCC_TARGET_X86_64 /* only works for x86-64 */ @@ -2070,7 +2073,8 @@ static int elf_output_file(TCCState *s1, const char *filename) x=s1->got->sh_addr - s1->plt->sh_addr - 12; p += 16; while (p < p_end) { - p += 4; + if (get32(p) == 0x46c04778) /* PLT Thumb stub present */ + p += 4; put32(p + 12, x + get32(p + 12) + s1->plt->data - p); p += 16; } @@ -2304,7 +2308,7 @@ static int elf_output_file(TCCState *s1, const char *filename) tcc_free(s1->symtab_to_dynsym); tcc_free(section_order); tcc_free(phdr); - tcc_free(s1->got_offsets); + tcc_free(s1->sym_attrs); return ret; } @@ -2599,6 +2603,18 @@ ST_FUNC int tcc_load_object_file(TCCState *s1, rel->r_info = ELFW(R_INFO)(sym_index, type); /* offset the relocation offset */ rel->r_offset += offseti; +#ifdef TCC_TARGET_ARM + /* Jumps and branches from a Thumb code to a PLT entry need + special handling since PLT entries are ARM code. + Unconditional bl instructions referencing PLT entries are + handled by converting these instructions into blx + instructions. Other case of instructions referencing a PLT + entry require to add a Thumb stub before the PLT entry to + switch to ARM mode. We set bit 0 of the got offset of a + symbol to indicate such a case. */ + if (type == R_ARM_THM_JUMP24) + alloc_sym_attr(s1, sym_index)->plt_thumb_stub = 1; +#endif } break; default: -- 2.11.4.GIT