SchOS/ldls/test/refmod.c

// SPDX-License-Identifier: Apache-2.0
//
// The loader's test object. **Ours, on purpose.**
//
// docs/architecture/ldls.md rule 1 says nothing GPL ever enters an image we build, and
// the same rule applies to the tree itself: there is no `.ko` committed here
// and there never will be. So the thing `make ldls-check` loads is this file,
// compiled with `-c`, which produces exactly what a `.ko` is -- an ELF64
// relocatable object for x86-64.
//
// It is written to contain one of each thing the loader has to get right,
// because a test object that only exercises the easy path is a test that
// passes on a loader that does not work:
//
//   - undefined symbols, of both the function and the data kind
//   - a `.text` with calls needing PC32/PLT32
//   - `.data` with an absolute pointer needing R_X86_64_64
//   - `.rodata` a pointer points into, so section-relative values matter
//   - `.bss`, which must arrive zeroed and is NOBITS with no file content
//   - a `.modinfo` shaped the way a module's is, so ko_modinfo() is exercised
//
// It is never linked and never run. Compiling it is the whole of its job.

// Things this object does not define. A real module's undefined list is
// hundreds of names from the kernel it was built against; five is enough to
// prove the loader reports them rather than to pretend this is a driver.
extern int   ldls_test_alloc(unsigned long size);
extern void  ldls_test_free(void *p);
extern int   ldls_test_print(const char *s);
extern int   ldls_test_counter;          // undefined *data*, not a function

// .rodata, and something in .data pointing at it -- which is the case that
// needs a section-relative symbol value to come out right.
static const char greeting[] = "ldls reference module";
const char *ldls_test_greeting = greeting;

// .bss. The loader must zero this rather than copy it, because NOBITS has no
// bytes in the file to copy.
static char scratch[256];
char *ldls_test_scratch = scratch;

// .data with a known value, so a test can check the bytes survived layout.
int ldls_test_magic = 0x1D15;

// .text with calls, which is where PC32 and PLT32 come from.
int
ldls_test_entry(void)
{
  int n = ldls_test_alloc(sizeof(scratch));

  if (n == 0)
    return -1;

  ldls_test_print(greeting);
  ldls_test_counter++;
  scratch[0] = (char)ldls_test_magic;
  ldls_test_free(scratch);
  return ldls_test_counter;
}

// A second function, called by the first, so at least one relocation resolves
// inside the object and does not need the resolver at all.
int
ldls_test_twice(void)
{
  return ldls_test_entry() + ldls_test_entry();
}

// Shaped like a module's `.modinfo`: NUL-terminated key=value strings in their
// own section. A real one is produced by MODULE_LICENSE() and friends; this is
// the same bytes by hand, because the macros that emit them are not ours.
#define MODINFO __attribute__((section(".modinfo"), used, aligned(1)))
MODINFO static const char mi_license[]     = "license=Apache-2.0";
MODINFO static const char mi_description[] = "description=LDLS loader test object";
MODINFO static const char mi_name[]        = "name=refmod";

// Modules define this; ko_name() looks for it to tell a kernel module from an
// ordinary object file.
int __this_module;

// The entry point, under the name `module_init(x)` gives it.
//
// L1 calls this, so it is written to exercise the thing L1 is for: a sequence
// of calls out to symbols this object does not define, in an order that can be
// checked. Three calls, always the same three, always in this order -- which
// is exactly what a real driver's init produces and what no amount of reading
// the symbol table would tell you.
//
// It deliberately does *not* dereference what the stubs return. A real driver
// does, faults, and that is a useful result; a test that faults every time is
// a test that cannot tell a fault it expected from one it did not.
int
init_module(void)
{
  ldls_test_print(greeting);
  ldls_test_alloc(sizeof(scratch));
  ldls_test_free(scratch);
  return 0;
}