SchOS/kernel/arch/x86_64/watch.c

// SPDX-License-Identifier: GPL-2.0-or-later
//
// Hardware watchpoints. docs/architecture/debugging.md milestone D4.
//
// The question this answers is the one no amount of printing gets at:
// *something wrote to this address -- who?* Adding a print to every writer
// only works when you already know who the writers are, and the bug is
// invariably a writer you have not thought of. The processor will simply tell
// you, through four debug registers that have been sitting unused in this
// kernel since it was written.
//
// Two decisions worth reading:
//
// **It reports and continues.** A data watchpoint on x86 is a *trap*, not a
// fault -- it is delivered after the storing instruction has completed -- so
// returning from the handler carries on with the write already done. That is
// what makes this usable on a running system, which docs/architecture/
// debugging.md argues is the whole point: the bugs here are timing bugs, and a
// debugger that stops the world cannot see them. The cost is that you cannot
// prevent the write, only learn about it.
//
// **Every CPU watches, without an IPI.** Debug registers are per-CPU, so
// arming DR0 on the processor that ran the command would watch one quarter of
// a four-way machine and silently miss the rest. There is no general IPI in
// this kernel, so instead the table carries a generation number and each CPU
// reloads its registers from the timer interrupt when it notices the number
// has moved. Worst case a watchpoint takes one tick to cover a CPU, and the
// cost when nothing has changed is one comparison per tick.

#include "types.h"
#include "defs.h"
#include "param.h"
#include "memlayout.h"
#include "mmu.h"
#include "x86.h"
#include "proc.h"
#include "traps.h"
#include "sbuf.h"
#include "dbg.h"

#define NWATCH 4                 // DR0..DR3, and there are no more

// DR7. The Intel manual's names, because nothing is gained by inventing new
// ones for a register that has had these since 1985.
#define DR7_L(n)      (1u << ((n) * 2))        // local enable
#define DR7_G(n)      (1u << ((n) * 2 + 1))    // global enable
#define DR7_LE        (1u << 8)
#define DR7_GE        (1u << 9)
#define DR7_RW_SHIFT(n)  (16 + (n) * 4)
#define DR7_LEN_SHIFT(n) (18 + (n) * 4)

#define RW_WRITE      0x1        // break on data write
#define RW_RW         0x3        // break on data read or write

struct watchpoint {
  uint64 addr;
  int    len;                    // 1, 2, 4 or 8
  int    rw;                     // RW_WRITE or RW_RW
  int    used;
  uint64 hits;
};

static struct watchpoint wp[NWATCH];
static volatile uint     wp_generation = 1;
static uint              wp_seen[NCPU];

// Recursion guard. The handler prints, and printing touches memory; a
// watchpoint armed on something cprintf writes to would otherwise re-enter
// here for ever. Per-CPU because two processors can be in the handler at once
// and one must not silence the other.
static int wp_busy[NCPU];

// --- the registers ----------------------------------------------------------

static inline uint64
read_dr6(void)
{
  uint64 v;
  asm volatile("mov %%dr6, %0" : "=r"(v));
  return v;
}

static inline void
write_dr6(uint64 v)
{
  asm volatile("mov %0, %%dr6" : : "r"(v));
}

static inline void
write_dr7(uint64 v)
{
  asm volatile("mov %0, %%dr7" : : "r"(v));
}

static void
write_dr(int n, uint64 v)
{
  switch (n) {
  case 0: asm volatile("mov %0, %%dr0" : : "r"(v)); break;
  case 1: asm volatile("mov %0, %%dr1" : : "r"(v)); break;
  case 2: asm volatile("mov %0, %%dr2" : : "r"(v)); break;
  case 3: asm volatile("mov %0, %%dr3" : : "r"(v)); break;
  }
}

// The LEN field is not a byte count. 1 byte is 00, 2 is 01, 8 is 10 and 4 is
// 11 -- the 8 and 4 encodings are the way round nobody expects, and getting
// them backwards produces a watchpoint that silently covers the wrong span.
static uint64
len_bits(int len)
{
  switch (len) {
  case 1:  return 0x0;
  case 2:  return 0x1;
  case 8:  return 0x2;
  default: return 0x3;           // 4
  }
}

// Load this CPU's debug registers from the table.
static void
wp_load(void)
{
  uint64 dr7 = DR7_LE | DR7_GE;

  for (int i = 0; i < NWATCH; i++) {
    if (!wp[i].used) {
      write_dr(i, 0);
      continue;
    }
    write_dr(i, wp[i].addr);
    // Global, not local. The local bits are cleared by a hardware task switch,
    // which this kernel never does -- but "watch this everywhere" is what is
    // meant, and G is the bit that says so.
    dr7 |= DR7_G(i);
    dr7 |= (uint64)wp[i].rw << DR7_RW_SHIFT(i);
    dr7 |= len_bits(wp[i].len) << DR7_LEN_SHIFT(i);
  }

  write_dr6(0);
  write_dr7(dr7);
}

// Called from the timer interrupt. Cheap when nothing has changed, which is
// every tick except the handful after somebody arms or clears a watchpoint.
void
watch_sync(void)
{
  int c = cpunum();

  if (c < 0 || c >= NCPU)
    return;
  if (wp_seen[c] == wp_generation)
    return;
  wp_seen[c] = wp_generation;
  wp_load();
}

// --- the handler ------------------------------------------------------------

// #DB. Say which watchpoint fired, where the write came from, and carry on.
//
// Returns 1 if this was ours, so trap() can tell a watchpoint apart from a
// single-step or any other source of #DB and not swallow the latter.
int
watch_trap(struct trapframe *tf)
{
  uint64 dr6 = read_dr6();
  int    c   = cpunum();
  int    hit = 0;

  if (!(dr6 & 0xF))
    return 0;                    // a #DB, but not one of ours

  write_dr6(0);                  // DR6 bits are sticky; nothing else clears them

  if (c >= 0 && c < NCPU) {
    if (wp_busy[c])
      return 1;                  // already reporting; see the note on recursion
    wp_busy[c] = 1;
  }

  for (int i = 0; i < NWATCH; i++) {
    addr_t pcs[10];
    char   where[72];

    if (!(dr6 & (1u << i)))
      continue;
    hit = 1;
    wp[i].hits++;

    ksym_fmt(where, sizeof(where), tf->rip);

    cprintf("watch %d: 0x%p written, %d byte(s), by %s (rip 0x%p) on cpu %d",
            i, wp[i].addr, wp[i].len, where, tf->rip, c);
    if (myproc())
      cprintf(", pid %d %s", myproc()->pid, myproc()->name);
    cprintf("\n");

    // The chain matters more than the rip. "memset wrote to it" is not an
    // answer; "memset, called from dma_alloc, called from do_dma" is.
    getstackpcs((addr_t *)tf->rbp, pcs);
    for (int j = 0; j < 6 && pcs[j]; j++) {
      ksym_fmt(where, sizeof(where), pcs[j]);
      cprintf("         #%d  %s\n", j, where);
    }
  }

  if (c >= 0 && c < NCPU)
    wp_busy[c] = 0;
  return hit;
}

// --- arming and clearing ----------------------------------------------------

static int
watch_set(uint64 addr, int len, int rw)
{
  int i;

  if (len != 1 && len != 2 && len != 4 && len != 8)
    return -1;
  // The address must be naturally aligned for the length. The processor
  // ignores the low bits otherwise and watches a span you did not ask for,
  // which is worse than refusing.
  if (addr & (uint64)(len - 1))
    return -2;

  for (i = 0; i < NWATCH; i++)
    if (!wp[i].used)
      break;
  if (i == NWATCH)
    return -3;

  wp[i].addr = addr;
  wp[i].len  = len;
  wp[i].rw   = rw;
  wp[i].hits = 0;
  wp[i].used = 1;

  wp_generation++;
  wp_load();                     // this CPU now; the rest at their next tick
  return i;
}

static int
watch_clear(int i)
{
  if (i < 0 || i >= NWATCH || !wp[i].used)
    return -1;
  wp[i].used = 0;
  wp_generation++;
  wp_load();
  return 0;
}

// --- the command ------------------------------------------------------------

static int
parse_num(const char **sp, uint64 *out)
{
  const char *s = *sp;
  uint64 v = 0;
  int any = 0, hex = 0;

  while (*s == ' ') s++;
  if (s[0] == '0' && (s[1] == 'x' || s[1] == 'X')) { hex = 1; s += 2; }
  else {
    const char *p = s;
    while (*p && *p != ' ') {
      if ((*p >= 'a' && *p <= 'f') || (*p >= 'A' && *p <= 'F')) { hex = 1; break; }
      p++;
    }
  }
  while (*s) {
    int d;
    if (*s >= '0' && *s <= '9')      d = *s - '0';
    else if (*s >= 'a' && *s <= 'f') d = *s - 'a' + 10;
    else if (*s >= 'A' && *s <= 'F') d = *s - 'A' + 10;
    else break;
    if (!hex && d > 9) break;
    v = hex ? (v << 4) + d : v * 10 + d;
    any = 1;
    s++;
  }
  *sp = s;
  *out = v;
  return any;
}

static void
watch_list(struct sbuf *out)
{
  int any = 0;

  sb_puts(out, "  #  address              len  on     hits\n");
  for (int i = 0; i < NWATCH; i++) {
    if (!wp[i].used)
      continue;
    any = 1;
    sb_putu_pad(out, i, 3);
    sb_puts(out, "  0x");
    sb_puthex(out, wp[i].addr, 16);
    sb_putu_pad(out, wp[i].len, 5);
    sb_puts(out, wp[i].rw == RW_RW ? "  rw   " : "  w    ");
    sb_putu_pad(out, wp[i].hits, 6);
    sb_putc(out, '\n');
  }
  if (!any)
    sb_puts(out, "  (none armed)\n");

  // Which processors have actually loaded the table.
  //
  // Reported rather than assumed, because the answer is the difference
  // between "this watchpoint is watching" and "this watchpoint is watching
  // one quarter of the machine". A CPU that is behind catches up at its next
  // timer tick; one that stays behind is a bug and this is where it shows.
  sb_puts(out, "\n  table generation ");
  sb_putu(out, wp_generation);
  sb_puts(out, ", loaded by:");
  for (int c = 0; c < ncpu && c < NCPU; c++) {
    sb_puts(out, "  cpu");
    sb_putu(out, c);
    sb_putc(out, '=');
    sb_putu(out, wp_seen[c]);
    if (wp_seen[c] != wp_generation)
      sb_puts(out, "(behind)");
  }
  sb_putc(out, '\n');

  sb_puts(out, "\n  watch <addr> [len] [w|rw]   arm one; len is 1, 2, 4 or 8\n"
               "  watch clear <n>             disarm one\n"
               "  watch clear all             disarm every one\n"
               "\n  Addresses are kernel virtual. `whose` prints the kernel va of a\n"
               "  physical page, which is how you get from one to the other.\n"
               "  Hits are reported to the console, not here -- they happen while\n"
               "  you are not looking, which is the point.\n");
}

static void
watch_view(struct sbuf *out, const char *arg)
{
  const char *p = arg;
  uint64 addr, len = 8;
  int rw = RW_WRITE, r;

  while (*p == ' ') p++;

  if (*p == 0) {
    watch_list(out);
    return;
  }

  if (p[0] == 'c' && p[1] == 'l') {              // clear
    while (*p && *p != ' ') p++;
    while (*p == ' ') p++;
    if (p[0] == 'a') {                           // all
      for (int i = 0; i < NWATCH; i++)
        if (wp[i].used)
          watch_clear(i);
      sb_puts(out, "all watchpoints disarmed\n");
      return;
    }
    if (!parse_num(&p, &addr)) {
      sb_puts(out, "watch clear <n>, or watch clear all\n");
      return;
    }
    if (watch_clear((int)addr) < 0)
      sb_puts(out, "no such watchpoint\n");
    else {
      sb_puts(out, "watch ");
      sb_putu(out, addr);
      sb_puts(out, " disarmed\n");
    }
    return;
  }

  if (!parse_num(&p, &addr)) {
    watch_list(out);
    return;
  }
  parse_num(&p, &len);                            // optional
  while (*p == ' ') p++;
  if (p[0] == 'r' && p[1] == 'w')
    rw = RW_RW;

  // A watchpoint on a user address would fire in whichever address space
  // happens to have that linear address mapped, which is every process. Say so
  // rather than arming something that reports nonsense.
  if (addr < (uint64)KERNBASE)
    sb_puts(out, "note: that is below KERNBASE, so it is a *user* linear address\n"
                 "      and will fire in whatever process is current. Kernel\n"
                 "      addresses are what this is for.\n");

  r = watch_set(addr, (int)len, rw);
  if (r == -1)      sb_puts(out, "length must be 1, 2, 4 or 8\n");
  else if (r == -2) sb_puts(out, "address must be aligned to its length\n");
  else if (r == -3) sb_puts(out, "all four debug registers are in use\n");
  else {
    sb_puts(out, "watch ");
    sb_putu(out, r);
    sb_puts(out, " armed on 0x");
    sb_puthex(out, addr, 16);
    sb_puts(out, ", ");
    sb_putu(out, len);
    sb_puts(out, rw == RW_RW ? " byte(s), read or write\n" : " byte(s), on write\n");
    sb_puts(out, "hits are reported to the console\n");
  }
}

void
watch_init(void)
{
  dbg_register_rw("watch", watch_view,
               "hardware watchpoints -- `watch <addr>` to arm one");
}