fix: handle mmap offset

add and use offset parameter in doMsync
add test case for shared mmap with offset

fixes emscripten-core#10094

AUTHORS

@@ -443,4 +443,5 @@ a license to everyone to use it as detailed in LICENSE.)
 * John Granström <[email protected]>
 * Clemens Backes <[email protected]> (copyright owned by Google, Inc.)
 * Tibor Klajnscek <[email protected]>
-* Benjamin Golinvaux <[email protected]>
+* Benjamin Golinvaux <[email protected]>
+* Peter Salomonsen <[email protected]>

src/library_syscall.js

@@ -69,9 +69,9 @@ var SyscallsLibrary = {
       {{{ makeSetValue('buf', C_STRUCTS.stat.st_ino, 'stat.ino', 'i64') }}};
       return 0;
     },
-    doMsync: function(addr, stream, len, flags) {
+    doMsync: function(addr, stream, len, flags, offset) {
       var buffer = new Uint8Array(HEAPU8.subarray(addr, addr + len));
-      FS.msync(stream, buffer, 0, len, flags);
+      FS.msync(stream, buffer, offset, len, flags);
     },
     doMkdir: function(path, mode) {
       // remove a trailing slash, if one - /a/b/ has basename of '', but
@@ -254,7 +254,7 @@ var SyscallsLibrary = {
       ptr = res.ptr;
       allocated = res.allocated;
     }
-    SYSCALLS.mappings[ptr] = { malloc: ptr, len: len, allocated: allocated, fd: fd, flags: flags };
+    SYSCALLS.mappings[ptr] = { malloc: ptr, len: len, allocated: allocated, fd: fd, flags: flags, offset: off };
     return ptr;
   },
 
@@ -272,7 +272,7 @@ var SyscallsLibrary = {
     if (!info) return 0;
     if (len === info.len) {
       var stream = FS.getStream(info.fd);
-      SYSCALLS.doMsync(addr, stream, len, info.flags);
+      SYSCALLS.doMsync(addr, stream, len, info.flags, info.offset);
       FS.munmap(stream);
       SYSCALLS.mappings[addr] = null;
       if (info.allocated) {
@@ -855,7 +855,7 @@ var SyscallsLibrary = {
     var addr = SYSCALLS.get(), len = SYSCALLS.get(), flags = SYSCALLS.get();
     var info = SYSCALLS.mappings[addr];
     if (!info) return 0;
-    SYSCALLS.doMsync(addr, FS.getStream(info.fd), len, info.flags);
+    SYSCALLS.doMsync(addr, FS.getStream(info.fd), len, info.flags, 0);
     return 0;
   },
   __syscall147__deps: ['$PROCINFO'],

tests/fs/test_mmap.c

@@ -126,5 +126,55 @@ int main() {
     fclose(fd);
   }
 
+  // MAP_SHARED with offset
+  {
+    const char* text = "written shared mmap with offset";
+    const char* path = "/yolo/sharedoffset.txt";
+
+    int fd = open(path, O_RDWR | O_CREAT | O_TRUNC, (mode_t)0600);
+    assert(fd != -1);
+
+    size_t textsize = strlen(text) + 1; // + \0 null character
+    // offset must be a multiple of the page size as returned by sysconf(_SC_PAGE_SIZE).
+    size_t offset = sysconf(_SC_PAGE_SIZE) * 2;
+
+    assert(lseek(fd, textsize + offset - 1, SEEK_SET) != -1);
+
+    // need to write something first to allow us to mmap
+    assert(write(fd, "", 1) != -1);
+
+    char *map;
+    map = (char*)mmap(0, textsize, PROT_READ | PROT_WRITE, MAP_SHARED, fd, offset - 1);
+    // assert failure if offset is not a multiple of page size
+    assert(map == MAP_FAILED);
+
+    map = (char*)mmap(0, textsize, PROT_READ | PROT_WRITE, MAP_SHARED, fd, offset);
+    assert(map != MAP_FAILED);
+
+    for (size_t i = 0; i < textsize; i++) {
+      map[i] = text[i];
+    }
+
+    assert(msync(map, textsize, MS_SYNC) != -1);
+
+    assert(munmap(map, textsize) != -1);
+
+    close(fd);
+  }
+
+  {
+    FILE* fd = fopen("/yolo/sharedoffset.txt", "r");
+    if (fd == NULL) {
+      printf("failed to open /yolo/sharedoffset.txt\n");
+      return 1;
+    }
+    size_t offset = sysconf(_SC_PAGE_SIZE) * 2;
+
+    char buffer[offset + 31];
+    fread(buffer, 1, offset + 32, fd);
+    // expect text written from mmap operation to appear at offset in the file
+    printf("/yolo/sharedoffset.txt content=%s %d\n", buffer + offset, offset);
+    fclose(fd);
+  }
   return 0;
 }

tests/fs/test_mmap.out

@@ -1,3 +1,4 @@
 /yolo/in.txt content=mmap ftw!
 /yolo/out.txt content=written mmap
 /yolo/private.txt content=
+/yolo/sharedoffset.txt content=written shared mmap with offset 32768