add a C version of wasi_hook

examples/wasi_hook_c/README.md

@@ -0,0 +1 @@
+C version of [wasi_hook](../wasi_hook).

examples/wasi_hook_c/app.c

@@ -0,0 +1,34 @@
+#include <assert.h>
+#include <sys/uio.h>
+
+#include <stdio.h>
+#include <unistd.h>
+
+#define bufsize 100
+
+int
+main()
+{
+        char buf[bufsize];
+        ssize_t len;
+
+        len = read(STDIN_FILENO, buf, sizeof(buf));
+        assert(len > 0);
+        printf("hello, \"%.*s\"\n", (int)len, buf);
+
+        char fub[bufsize];
+        struct iovec iov[sizeof(fub)];
+        ssize_t len2;
+        int i;
+        for (i = 0; i < sizeof(fub); i++) {
+                iov[i].iov_base = &fub[sizeof(fub) - 1 - i];
+                iov[i].iov_len = 1;
+        }
+        len2 = readv(STDIN_FILENO, iov, sizeof(iov));
+        assert(len2 == len);
+        /* skip NUL */
+        assert(fub[sizeof(fub) - len2] == 0);
+        len2--;
+        printf("hello, \"%.*s\" (reversed)\n", (int)len2,
+               &fub[sizeof(fub) - len2]);
+}

examples/wasi_hook_c/hook-asm.S

@@ -0,0 +1,64 @@
+    .text
+
+    .tabletype __indirect_function_table, funcref
+    .globaltype __stack_pointer, i32
+
+    .functype my_fd_read (i32,i32,i32,i32) -> (i32)
+    .functype init_C () -> ()
+
+    .functype pivot_set_func (i32,funcref) -> ()
+    .import_module pivot_set_func,pivot
+    .import_name pivot_set_func,set_func
+
+    .globaltype sp, i32
+    .globaltype mypage, i32
+sp:
+mypage:
+
+    .globl _initialize
+    .type _initialize,@function
+
+    .globl set_func
+    .type set_func,@function
+
+_initialize:
+    .functype _initialize () -> ()
+    .export_name _initialize,_initialize
+
+    # set up C context for this module.
+    #
+    # steal a page via memory.grow.
+    # this requires the main module prepared for this.
+    # cf. https://github.com/WebAssembly/wasi-libc/pull/377
+    #
+    # todo: tls
+
+    .local i32
+
+    i32.const 1
+    memory.grow 0
+    local.tee 0
+    i32.const -1
+    i32.eq
+    if
+      unreachable
+    end_if
+    local.get 0
+    global.set mypage
+    local.get 0
+    i32.const 1
+    i32.add
+    i32.const 65536
+    i32.mul
+    global.set sp
+
+    call init_C
+    end_function
+
+set_func:
+    .functype set_func (i32, i32) -> ()
+    local.get 0
+    local.get 1
+    table.get __indirect_function_table
+    call pivot_set_func
+    end_function

examples/wasi_hook_c/hook-c.c

@@ -0,0 +1,65 @@
+#include <string.h>
+#include <sys/uio.h>
+
+/*
+ * CAVEAT: only very limited C environment is available here.
+ *
+ * - no data/bss (as it overlaps with the main app)
+ * - no tls
+ */
+
+int
+my_fd_read(int fd, struct iovec *iov, size_t iovlen, size_t *retp)
+{
+        /*
+         * this function does something unnecessarily complex to ensure
+         * using C stack
+         */
+
+        char buf[10];
+
+        /*
+         * we avoid using string literals because
+         * they are usually placed in (ro)data section.
+         * we assume that the compiler is not smart enough to
+         * turn this into a single memcpy from rodata.
+         *
+         * note: linear memory is little endian.
+         */
+        unsigned int meow = 0x776f654d;
+        unsigned int in_c = 0x43206e69;
+        memcpy(buf, &meow, 4);
+        buf[4] = ' ';
+        memcpy(&buf[5], &in_c, 4);
+        buf[9] = 0;
+
+        const char *cp = buf;
+        size_t len = strlen(cp) + 1;
+        while (iovlen > 0) {
+                size_t n;
+                if (len > iov->iov_len) {
+                        n = iov->iov_len;
+                } else {
+                        n = len;
+                }
+                memcpy(iov->iov_base, cp, n);
+                cp += n;
+                len -= n;
+                iov++;
+                iovlen--;
+        }
+        *retp = cp - buf;
+        return 0;
+}
+
+void
+init_C()
+{
+        void set_func(int, void *);
+
+        /*
+         * the first argument of set_func is the index in $func-table
+         * in pivot.wat.
+         */
+        set_func(0, my_fd_read);
+}

examples/wasi_hook_c/pivot.wat

@@ -0,0 +1,17 @@
+(module
+  (type $fd_read_type (func (param i32 i32 i32 i32) (result i32)))
+  (func (export "fd_read") (type $fd_read_type)
+    local.get 0
+    local.get 1
+    local.get 2
+    local.get 3
+    i32.const 0
+    call_indirect $func-table (type $fd_read_type)
+  )
+  (func (export "set_func") (param i32 funcref)
+    local.get 0
+    local.get 1
+    table.set $func-table
+  )
+  (table $func-table 1 1 funcref)
+)

examples/wasi_hook_c/repl.txt

@@ -0,0 +1,11 @@
+:load pivot.wasm
+:register pivot
+:register wasi_snapshot_preview1
+
+:module app load app.wasm
+:register app
+
+:load hook.wasm
+:invoke _initialize
+
+:module app invoke _start

examples/wasi_hook_c/run.sh

@@ -0,0 +1,24 @@
+#! /bin/sh
+
+set -e
+
+CC=${CC:-/opt/wasi-sdk-20.0/bin/clang}
+TOYWASM=${TOYWASM:-toywasm}
+
+wat2wasm pivot.wat
+
+${CC} -o app.wasm app.c
+
+CFLAGS="-O3 -mreference-types -Wall -Werror"
+${CC} -c ${CFLAGS} hook-asm.S
+${CC} -c ${CFLAGS} hook-c.c
+# note: --import-memory makes the linker to use __wasm_init_memory
+${CC} -v \
+-Wl,--no-entry \
+-Wl,--export=__wasm_call_ctors \
+-Xlinker --import-memory=app,memory \
+-nostdlib \
+-o hook.wasm hook-asm.o hook-c.o -lc
+
+${TOYWASM} --trace=5 --wasi --repl < repl.txt
+#${TOYWASM} --wasi --repl < repl.txt