139 lines
3.8 KiB
C
139 lines
3.8 KiB
C
/*
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* Layout (file order) without .data:
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* [0..64) ELF header
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* [64..120) one program header (PT_LOAD R+X)
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* [120..0x1000) zero pad
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* [0x1000..) .text bytes
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*
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* With .data (any input .o has writable globals):
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* [0..64) ELF header
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* [64..176) two program headers (PT_LOAD R+X, PT_LOAD R+W)
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* [176..0x1000) zero pad
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* [0x1000..) .text bytes
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* [data_off..) .data bytes (file offset and vaddr page-aligned)
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*/
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#include "l.h"
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#define ET_EXEC 2
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#define EM_X86_64 62
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#define EV_CURRENT 1
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#define ELFCLASS64 2
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#define ELFDATA2LSB 1
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#define PT_LOAD 1
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#define PF_X 1
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#define PF_W 2
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#define PF_R 4
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#pragma pack(push, 1)
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typedef struct {
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u8 e_ident[16];
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u16 e_type, e_machine;
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u32 e_version;
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u64 e_entry, e_phoff, e_shoff;
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u32 e_flags;
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u16 e_ehsize, e_phentsize, e_phnum, e_shentsize, e_shnum, e_shstrndx;
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} Ehdr;
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typedef struct {
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u32 p_type, p_flags;
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u64 p_offset, p_vaddr, p_paddr;
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u64 p_filesz, p_memsz, p_align;
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} Phdr;
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#pragma pack(pop)
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int
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l_emit_elf(Lnk *l, FILE *f, u64 base, u64 entry)
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{
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/* Dispatch: any loaded shared object plus any dynamic ref means
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* we owe the loader a real PT_INTERP/PT_DYNAMIC binary. */
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if (l->sos != NULL && l->dyn_n > 0)
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return l_emit_dyn_elf(l, f, base, entry);
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const u64 page = 0x1000;
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const u64 text_off = 0x1000;
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const u64 rx_end = text_off + l->textlen;
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const int has_data = (l->datalen > 0);
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/* data goes at the next page boundary so the loader can grant a
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* fresh page of R+W permissions without overlapping the R+X mapping. */
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const u64 data_off = has_data ? ((rx_end + page - 1) & ~(page - 1)) : 0;
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const u64 data_va = has_data ? (base + data_off) : 0;
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/* Apply relocations now that the layout's text_va/data_va are
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* known. Deferred from main.c so the dynamic-link path can use
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* its own data_va. */
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if (l_relocate(l, base + text_off, data_va) != 0) return 1;
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/* BSS optimisation: trailing zero bytes in .data can be left out
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* of the file. The loader zero-fills the gap between p_filesz and
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* p_memsz, so this shrinks the binary without changing semantics.
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* Scan after l_relocate has applied DATAR patches — anything still
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* zero at the tail genuinely is zero-init. */
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u64 bsslen = 0;
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if (has_data) {
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while (bsslen < l->datalen
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&& l->data[l->datalen - 1 - bsslen] == 0)
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bsslen++;
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}
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const u64 data_file_len = l->datalen - bsslen;
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Ehdr eh = {0};
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memcpy(eh.e_ident, "\x7f""ELF", 4);
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eh.e_ident[4] = ELFCLASS64;
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eh.e_ident[5] = ELFDATA2LSB;
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eh.e_ident[6] = EV_CURRENT;
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eh.e_type = ET_EXEC;
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eh.e_machine = EM_X86_64;
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eh.e_version = EV_CURRENT;
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eh.e_entry = entry;
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eh.e_phoff = sizeof(Ehdr);
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eh.e_ehsize = sizeof(Ehdr);
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eh.e_phentsize = sizeof(Phdr);
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eh.e_phnum = has_data ? 2 : 1;
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Phdr phx = {0};
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phx.p_type = PT_LOAD;
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phx.p_flags = PF_R | PF_X;
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phx.p_offset = 0;
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phx.p_vaddr = base;
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phx.p_paddr = base;
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phx.p_filesz = rx_end;
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/* The loader page-rounds mappings; when present, data_off starts
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* independently on the next page, so padding is not R+X extent. */
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phx.p_memsz = rx_end;
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phx.p_align = page;
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Phdr phw = {0};
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if (has_data) {
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phw.p_type = PT_LOAD;
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phw.p_flags = PF_R | PF_W;
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phw.p_offset = data_off;
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phw.p_vaddr = base + data_off;
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phw.p_paddr = base + data_off;
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phw.p_filesz = data_file_len;
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phw.p_memsz = l->datalen;
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phw.p_align = page;
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}
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fwrite(&eh, 1, sizeof eh, f);
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fwrite(&phx, 1, sizeof phx, f);
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if (has_data) fwrite(&phw, 1, sizeof phw, f);
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long here = ftell(f);
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for (long i = here; i < (long)text_off; i++) fputc(0, f);
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if (l->textlen) fwrite(l->text, 1, l->textlen, f);
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if (has_data && data_file_len > 0) {
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here = ftell(f);
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for (long i = here; i < (long)data_off; i++) fputc(0, f);
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fwrite(l->data, 1, data_file_len, f);
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}
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return 0;
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}
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