SchOS/ldls/ko.h

// SPDX-License-Identifier: GPL-2.0-or-later
#pragma once

// Loading a relocatable object. docs/architecture/ldls.md, "The loader".
//
// A Linux `.ko` is an ordinary ELF64 relocatable object -- the same thing
// `gcc -c` produces -- and loading one is four steps with no mystery in any of
// them: index the sections, place the allocatable ones, resolve the undefined
// symbols against a table, and apply the relocations. There is no licence
// question anywhere in this file: ELF and the x86-64 relocation set are
// published specifications, which is the case [../docs/licensing.md] §5
// describes.
//
// **It is freestanding on purpose.** No libc, no allocation, no I/O: the
// caller hands over the whole image and a buffer, and everything happens in
// them. That is what lets the same source build as a host tool -- where a test
// takes a second -- and as the SchOS-side host, which is the discipline
// `mkschfs` and the kernel already share and the reason SchFS format bugs are
// found on the host.
//
// Types are spelled out rather than taken from a header, because the two sides
// have different ones and this file has to mean the same thing in both.

// The section table is the caller's, not ours.
//
// A fixed array needs a limit, and every limit that would fit in a struct is
// one a real object exceeds: measured over 300 modules the median is 50
// sections and the 95th percentile is 67, but an object built with
// -ffunction-sections runs to thousands -- nvidia-modeset has 7,069. Rather
// than pick a number that is generous until it is not, the caller says how
// many it is prepared for, and ko_sections_needed() says how many that is.
//
// A suggested size for a fixed table on the SchOS side, where allocating is
// less convenient. It is a caller's choice and not the loader's limit.
#define KO_SECHINT  128

// Undefined symbols the report will hold. Measured over the same corpus the
// median is 25 and the worst seen is 245, so this is room to spare -- and
// `undef_lost` says so rather than the list quietly ending.
#define KO_MAXUNDEF 512

// --- ELF64, the relocatable subset ------------------------------------------
//
// include/elf.h has `elfhdr` and `proghdr`, which are the ET_EXEC half. None of
// what a relocatable object needs is there, so it is here.

struct ko_ehdr {
  unsigned char      e_ident[16];
  unsigned short     e_type;
  unsigned short     e_machine;
  unsigned int       e_version;
  unsigned long long e_entry;
  unsigned long long e_phoff;
  unsigned long long e_shoff;
  unsigned int       e_flags;
  unsigned short     e_ehsize;
  unsigned short     e_phentsize;
  unsigned short     e_phnum;
  unsigned short     e_shentsize;
  unsigned short     e_shnum;
  unsigned short     e_shstrndx;
};

struct ko_shdr {
  unsigned int       sh_name;
  unsigned int       sh_type;
  unsigned long long sh_flags;
  unsigned long long sh_addr;
  unsigned long long sh_offset;
  unsigned long long sh_size;
  unsigned int       sh_link;
  unsigned int       sh_info;
  unsigned long long sh_addralign;
  unsigned long long sh_entsize;
};

struct ko_sym {
  unsigned int       st_name;
  unsigned char      st_info;
  unsigned char      st_other;
  unsigned short     st_shndx;
  unsigned long long st_value;
  unsigned long long st_size;
};

struct ko_rela {
  unsigned long long r_offset;
  unsigned long long r_info;
  long long          r_addend;
};

#define KO_ET_REL      1
#define KO_EM_X86_64   62

#define KO_SHT_PROGBITS 1
#define KO_SHT_SYMTAB   2
#define KO_SHT_STRTAB   3
#define KO_SHT_RELA     4
#define KO_SHT_NOBITS   8

#define KO_SHF_WRITE     0x1
#define KO_SHF_ALLOC     0x2
#define KO_SHF_EXECINSTR 0x4

#define KO_SHN_UNDEF   0
#define KO_SHN_ABS     0xfff1
#define KO_SHN_COMMON  0xfff2

#define KO_R_SYM(i)  ((unsigned int)((i) >> 32))
#define KO_R_TYPE(i) ((unsigned int)((i) & 0xffffffffu))

// The x86-64 relocations a compiled object actually contains. Anything else is
// refused by name rather than ignored -- a relocation silently skipped is a
// wrong address found much later.
#define KO_R_X86_64_NONE   0
#define KO_R_X86_64_64     1
#define KO_R_X86_64_PC32   2
#define KO_R_X86_64_PLT32  4
#define KO_R_X86_64_32    10
#define KO_R_X86_64_32S   11
#define KO_R_X86_64_PC64  24

// --- what the loader knows about a section ----------------------------------

struct ko_section {
  const char         *name;
  unsigned int        type;
  unsigned long long  flags;
  unsigned long long  size;
  unsigned long long  align;
  const unsigned char *data;    // 0 for NOBITS
  int                 alloc;    // takes space at run time
  unsigned long long  addr;     // where it will run, once laid out
  unsigned char      *where;    // where it is right now, once laid out
};

// Resolving a name to an address. Returns 0 and fills `out` on success, or
// non-zero to say it does not know it -- which is not an error at L0, it is
// the answer we are asking for.
typedef int (*ko_resolver)(void *ctx, const char *name,
                           unsigned long long *out);

struct ko {
  const unsigned char *img;
  unsigned long long   len;

  const struct ko_ehdr *eh;
  const unsigned char  *shtab;      // raw, indexed with e_shentsize
  int                   nsh;

  const char           *shstr;      // section name strings
  const struct ko_sym  *sym;
  int                   nsym;
  const char           *str;        // symbol name strings

  struct ko_section *sec;           // the caller's, nsh entries used

  const char *undef[KO_MAXUNDEF];
  int         nundef;
  int         undef_lost;           // more than KO_MAXUNDEF of them

  unsigned long long span;          // bytes ko_layout needs
  int                laid_out;

  // How many relocations of each type the object contains, counted by
  // ko_reltypes(). The question "what does the loader actually have to
  // handle" has an exact answer per module, and guessing at it is how a
  // loader ends up refusing a driver on its last relocation.
  unsigned int       reltype[64];
  unsigned int       reltype_other;
  unsigned int       nrel;

  char err[128];
};

// How many `struct ko_section` this object needs. -1 if it is not an ELF64
// object at all, which is worth knowing before allocating anything for it.
int ko_sections_needed(const void *image, unsigned long long len);

// Validate and index, using `sec` for the section table. Returns 0, or -1 with
// ko_err() set -- including when `nsec` is too small, which says how many were
// needed rather than just refusing.
int ko_open(struct ko *k, const void *image, unsigned long long len,
            struct ko_section *sec, int nsec);

// Bytes ko_layout will need. Valid after ko_open.
unsigned long long ko_span(const struct ko *k);

// Place every allocatable section into `mem`, as if it will run at `base`.
// The two differ when the object is staged somewhere before being mapped;
// pass base == (address of mem) when they do not.
int ko_layout(struct ko *k, void *mem, unsigned long long cap,
              unsigned long long base);

// Count the relocations by type, without applying any. Valid after ko_open.
void ko_reltypes(struct ko *k);

// Apply every relocation. Undefined symbols go to `r`; a name it does not know
// makes this fail, naming the first one.
int ko_relocate(struct ko *k, ko_resolver r, void *ctx);

// Neutralise every `%gs:`-relative load of a named symbol.
//
// Linux builds modules with `-mstack-protector-guard-reg=gs`, so the stack
// canary is fetched through the GS segment:
//
//     mov %gs:__ref_stack_chk_guard(%rip), %rax     (prologue)
//     sub %gs:__ref_stack_chk_guard(%rip), %rax     (epilogue, then jne)
//
// That is an ABI between the kernel and its own per-CPU area, and a host which
// is not that kernel cannot provide it. On SchOS the GS base belongs to the
// kernel, so a user-mode read of it faults on a kernel address -- which is
// exactly where 8139too's probe stopped the first time it ran there.
//
// So the instruction is rewritten to zero the destination register. The canary
// becomes zero, the check at the end of the function compares zero against
// zero, and `__stack_chk_fail` is never reached. What is lost is a hardening
// measure that was never going to work here; what is gained is a driver that
// runs.
//
// Call it **after** ko_relocate(), which will have written a displacement into
// bytes this then overwrites entirely. Returns how many it patched, or -1 if
// it found something at that offset it did not recognise -- in which case
// nothing is modified, because a loader that half-understands an instruction
// should not be editing it.
int ko_defuse_gs(struct ko *k, const char *sym);

// Turn every *other* `%gs:`-relative reference into an ordinary one.
//
// The stack guard is one per-CPU variable; there are others, and `current` is
// the one every driver touches. `mov %gs:const_current_task(%rip), %r8` reads
// a pointer out of the kernel's per-CPU area, and a host that is not that
// kernel has no such area -- so on SchOS it reads a kernel address from user
// mode and faults.
//
// Defusing is not available here: the guard's *value* is arbitrary, so zeroing
// a register is a correct answer, and `current`'s is not -- the driver will
// dereference it.
//
// What works instead is that **a per-CPU variable in a single-threaded host is
// just a variable**. Taking the `%gs` off leaves the same instruction reading
// the same width in the same direction, from storage the embedder supplies for
// that name. Eight bytes become seven and a nop, and the driver never learns
// the difference.
//
// `percpu` is asked for an address for each name and should return the same
// one every time it is asked for that name -- and should initialise it, since
// the driver now reads its *contents*: zero is right for a counter and wrong
// for `current`. Returning 0 refuses the object.
//
// Call after ko_relocate() and after ko_defuse_gs(), for the same reason.
// Returns how many it rewrote, or -1.
int ko_fixup_percpu(struct ko *k,
                    unsigned long long (*percpu)(void *ctx, const char *name),
                    void *ctx);

// A value from the `.modinfo` section -- "license", "description", "author",
// "alias". Returns 0 if the object has no such key.
//
// Worth having at L0 rather than later: the first thing anyone should be able
// to ask about a module is what licence it declares, and that is a string in
// the file rather than anything that needs loading.
const char *ko_modinfo(const struct ko *k, const char *key);

// The module's own name, from `.gnu.linkonce.this_module`, or 0.
const char *ko_name(const struct ko *k);

const char *ko_err(const struct ko *k);