Cut dynamic-linker startup syscalls

The dynamic-linker bring-up storm was the largest remaining startup band
after pull request #34. Adding a per-syscall histogram pointed at the
sidecar walker as the openat dominant cost (61% of getent startup) and
the per-call path_translation_t memset as the second source.

src/debug/syscall-hist.[ch]: opt-in histogram via
ELFUSE_STARTUP_TRACE=syscalls (or =all alongside the existing step
trace). Lock-free atomic counters per Linux syscall number, sorted
total-ns descending in the dump. Records freeze on the first successful
execve so steady-state traffic does not pollute the startup picture.
Fork children disable the histogram explicitly because they resume from
a parent snapshot, not a fresh bring-up.

src/syscall/sidecar.c: First a per-directory absence cache keyed by
(st_dev, st_ino, mtime, ctime) so the walker can skip the openat for
.elfuse-sidecar-index when a recent fstat on the same dirfd already saw
ENOENT. The mtime/ctime in the key closes ABA naturally and makes a
cross-process index publish observable without explicit invalidation.
Second a cached sysroot dirfd handed out as fcntl(F_DUPFD_CLOEXEC, 0) so
each translated absolute path saves the ~30 us open(sysroot) round-trip
and the dup carries CLOEXEC across any racing posix_spawn.

src/syscall/path.c: drop the per-call zero-init of path_translation_t.
The struct is ~12 KiB (24 metadata bytes plus three LINUX_PATH_MAX
buffers) and the buffers are read-after-written by their respective
resolvers. memset of all three was the dominant remaining cost after the
sidecar caches.

src/core/elf.c: skip the redundant memset of the file-data range in
elf_map_segments. The loader previously zeroed the full page-aligned
segment extent before issuing fread; now only the BSS portion plus page
padding (filesz to zero_len) is zeroed.

src/core/startup-trace.h: env parsing extended to comma-separated tokens
(steps, syscalls, all); legacy =1 keeps enabling steps only so existing
scripts keep working.

Measurement: 30-run distributions under ELFUSE_STARTUP_TRACE=syscalls,
warm cache:
  bench-hot-guard-glibc startup syscalls:
    5.225 ms baseline (single sample) -> 1.33 ms p50
    (p25 1.21, p75 1.55, stdev 0.45, n=30)         3.9x
  bench openat per-call:
    135 us baseline -> 33.4 us p50
    (p25 32.4, p75 35.8, stdev 7.1, n=30)          4.0x
  getent passwd root startup syscalls:
    7.478 ms baseline -> 2.22 ms p50
    (p25 2.10, p75 2.28, stdev 0.27, n=30)         3.4x
  getent openat per-call:
    230 us baseline -> 52.9 us p50
    (p25 51.5, p75 55.1, stdev 2.2, n=30)          4.3x

End-to-end wall-clock for getent: 14.6 ms p50 (p25 14.3, p75 15.1, stdev
1.18, n=30). Bench guardrail steady-state: static getpid 74 ns,
clock_gettime 6.7 ns, urandom1 153 ns; dynamic-glibc getpid 53 ns,
clock_gettime 6.4 ns, urandom1 142 ns. All under ceilings.

The original baselines were single first-run samples; their variance
band was not measured, so the speedup ratios are best-effort relative
to the cited starting point.

Author: jserv

Makefile

@@ -66,7 +66,8 @@ SRCS := \
     debug/gdbstub.c \
     debug/gdbstub-reg.c \
     debug/gdbstub-rsp.c \
-    debug/log.c
+    debug/log.c \
+    debug/syscall-hist.c
 
 SRCS := $(addprefix src/,$(SRCS))
 OBJS := $(patsubst src/%.c,$(BUILD_DIR)/%.o,$(SRCS))

src/core/elf.c

@@ -348,18 +348,16 @@ int elf_map_segments(const elf_info_t *info,
             return -1;
         }
 
-        /* Zero the full page-aligned segment extent (zero_len computed above
-         * with guest_size and infra_reserve checks). Linux guarantees
-         * zero-filled tail bytes in the last mapped page, and some dynamic
-         * linkers allocate from that page tail before they request more
-         * memory. Leaving stale bytes there leaks state across execve and
-         * corrupts the new image.
+        /* Zero only the tail beyond filesz: the BSS portion [filesz, memsz)
+         * plus the page-padding [memsz, zero_len) that Linux guarantees clean
+         * for dynamic linkers allocating from the last mapped page's tail.
+         * Skipping the file-data range avoids writing zeros that the fread
+         * below would immediately overwrite; for typical shared libraries that
+         * is a hundreds-of-KiB win per segment.
          */
-        memset((uint8_t *) guest_base + gpa, 0, zero_len);
+        if (zero_len > filesz)
+            memset((uint8_t *) guest_base + gpa + filesz, 0, zero_len - filesz);
 
-        /* Overlay initialized bytes after zeroing so BSS and page tail remain
-         * zero-filled.
-         */
         if (filesz > 0) {
             if (fseek(f, (long) ph->p_offset, SEEK_SET) != 0) {
                 log_error("%s: seek failed for segment at 0x%llx", path,

src/core/startup-trace.h

@@ -9,6 +9,15 @@
  * static inline so each translation unit can use them without pulling in a
  * separate object; the getenv check resolves once per translation unit but
  * the resolution itself is idempotent.
+ *
+ * Accepted env values:
+ *   unset, "", "0"  -> all tracing off
+ *   "1", "steps"    -> per-step VM bring-up timings (this header)
+ *   "syscalls"      -> per-syscall histogram (debug/syscall-hist.c)
+ *   "all"           -> both, comma-separated tokens also accepted
+ * "1" is preserved as a legacy alias for "steps" so old scripts keep
+ * working. The histogram mode never enables the step tracer and vice
+ * versa, so a user can ask for one without paying for the other.
  */
 
 #ifndef ELFUSE_STARTUP_TRACE_H
@@ -30,10 +39,37 @@
 static pthread_once_t startup_trace_once = PTHREAD_ONCE_INIT;
 static bool startup_trace_value;
 
+static inline bool startup_trace_env_has(const char *env, const char *tok)
+{
+    if (!env || !env[0])
+        return false;
+    size_t toklen = strlen(tok);
+    const char *p = env;
+    while (*p) {
+        const char *comma = strchr(p, ',');
+        size_t len = comma ? (size_t) (comma - p) : strlen(p);
+        if (len == toklen && memcmp(p, tok, toklen) == 0)
+            return true;
+        if (!comma)
+            break;
+        p = comma + 1;
+    }
+    return false;
+}
+
 static inline void startup_trace_resolve(void)
 {
     const char *v = getenv("ELFUSE_STARTUP_TRACE");
-    startup_trace_value = v && v[0] && strcmp(v, "0") != 0;
+    if (!v || !v[0] || !strcmp(v, "0"))
+        return;
+    /* The legacy "1" knob enables steps. Recognize it both as the whole
+     * value and as a token so compound forms like "1,syscalls" still
+     * keep the step trace on alongside the histogram, instead of
+     * silently dropping it.
+     */
+    if (!strcmp(v, "1") || startup_trace_env_has(v, "1") ||
+        startup_trace_env_has(v, "steps") || startup_trace_env_has(v, "all"))
+        startup_trace_value = true;
 }
 
 static inline bool startup_trace_enabled(void)