ftrace/scripts: Have recordmcount copy the object file

commit a50bd439 upstream.

Russell King found that he had weird side effects when compiling the kernel
with hard linked ccache. The reason was that recordmcount modified the
kernel in place via mmap, and when a file gets modified twice by
recordmcount, it will complain about it. To fix this issue, Russell wrote a
patch that checked if the file was hard linked more than once and would
unlink it if it was.

Linus Torvalds was not happy with the fact that recordmcount does this in
place modification. Instead of doing the unlink only if the file has two or
more hard links, it does the unlink all the time. In otherwords, it always
does a copy if it changed something. That is, it does the write out if a
change was made.
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

scripts/recordmcount.c

@@ -42,12 +42,17 @@
 
 static int fd_map;	/* File descriptor for file being modified. */
 static int mmap_failed; /* Boolean flag. */
-static void *ehdr_curr; /* current ElfXX_Ehdr *  for resource cleanup */
 static char gpfx;	/* prefix for global symbol name (sometimes '_') */
 static struct stat sb;	/* Remember .st_size, etc. */
 static jmp_buf jmpenv;	/* setjmp/longjmp per-file error escape */
 static const char *altmcount;	/* alternate mcount symbol name */
 static int warn_on_notrace_sect; /* warn when section has mcount not being recorded */
+static void *file_map;	/* pointer of the mapped file */
+static void *file_end;	/* pointer to the end of the mapped file */
+static int file_updated; /* flag to state file was changed */
+static void *file_ptr;	/* current file pointer location */
+static void *file_append; /* added to the end of the file */
+static size_t file_append_size; /* how much is added to end of file */
 
 /* setjmp() return values */
 enum {
@@ -61,10 +66,14 @@ static void
 cleanup(void)
 {
 	if (!mmap_failed)
-		munmap(ehdr_curr, sb.st_size);
+		munmap(file_map, sb.st_size);
 	else
-		free(ehdr_curr);
-	close(fd_map);
+		free(file_map);
+	file_map = NULL;
+	free(file_append);
+	file_append = NULL;
+	file_append_size = 0;
+	file_updated = 0;
 }
 
 static void __attribute__((noreturn))
@@ -86,12 +95,22 @@ succeed_file(void)
 static off_t
 ulseek(int const fd, off_t const offset, int const whence)
 {
-	off_t const w = lseek(fd, offset, whence);
-	if (w == (off_t)-1) {
-		perror("lseek");
+	switch (whence) {
+	case SEEK_SET:
+		file_ptr = file_map + offset;
+		break;
+	case SEEK_CUR:
+		file_ptr += offset;
+		break;
+	case SEEK_END:
+		file_ptr = file_map + (sb.st_size - offset);
+		break;
+	}
+	if (file_ptr < file_map) {
+		fprintf(stderr, "lseek: seek before file\n");
 		fail_file();
 	}
-	return w;
+	return file_ptr - file_map;
 }
 
 static size_t
@@ -108,12 +127,38 @@ uread(int const fd, void *const buf, size_t const count)
 static size_t
 uwrite(int const fd, void const *const buf, size_t const count)
 {
-	size_t const n = write(fd, buf, count);
-	if (n != count) {
-		perror("write");
-		fail_file();
+	size_t cnt = count;
+	off_t idx = 0;
+
+	file_updated = 1;
+
+	if (file_ptr + count >= file_end) {
+		off_t aoffset = (file_ptr + count) - file_end;
+
+		if (aoffset > file_append_size) {
+			file_append = realloc(file_append, aoffset);
+			file_append_size = aoffset;
+		}
+		if (!file_append) {
+			perror("write");
+			fail_file();
+		}
+		if (file_ptr < file_end) {
+			cnt = file_end - file_ptr;
+		} else {
+			cnt = 0;
+			idx = aoffset - count;
+		}
 	}
-	return n;
+
+	if (cnt)
+		memcpy(file_ptr, buf, cnt);
+
+	if (cnt < count)
+		memcpy(file_append + idx, buf + cnt, count - cnt);
+
+	file_ptr += count;
+	return count;
 }
 
 static void *
@@ -170,9 +215,7 @@ static int make_nop_x86(void *map, size_t const offset)
  */
 static void *mmap_file(char const *fname)
 {
-	void *addr;
-
-	fd_map = open(fname, O_RDWR);
+	fd_map = open(fname, O_RDONLY);
 	if (fd_map < 0 || fstat(fd_map, &sb) < 0) {
 		perror(fname);
 		fail_file();
@@ -181,29 +224,58 @@ static void *mmap_file(char const *fname)
 		fprintf(stderr, "not a regular file: %s\n", fname);
 		fail_file();
 	}
-	addr = mmap(0, sb.st_size, PROT_READ|PROT_WRITE, MAP_PRIVATE,
-		    fd_map, 0);
+	file_map = mmap(0, sb.st_size, PROT_READ|PROT_WRITE, MAP_PRIVATE,
+			fd_map, 0);
 	mmap_failed = 0;
-	if (addr == MAP_FAILED) {
+	if (file_map == MAP_FAILED) {
 		mmap_failed = 1;
-		addr = umalloc(sb.st_size);
-		uread(fd_map, addr, sb.st_size);
+		file_map = umalloc(sb.st_size);
+		uread(fd_map, file_map, sb.st_size);
 	}
-	if (sb.st_nlink != 1) {
-		/* file is hard-linked, break the hard link */
-		close(fd_map);
-		if (unlink(fname) < 0) {
-			perror(fname);
-			fail_file();
-		}
-		fd_map = open(fname, O_RDWR | O_CREAT, sb.st_mode);
-		if (fd_map < 0) {
-			perror(fname);
+	close(fd_map);
+
+	file_end = file_map + sb.st_size;
+
+	return file_map;
+}
+
+static void write_file(const char *fname)
+{
+	char tmp_file[strlen(fname) + 4];
+	size_t n;
+
+	if (!file_updated)
+		return;
+
+	sprintf(tmp_file, "%s.rc", fname);
+
+	/*
+	 * After reading the entire file into memory, delete it
+	 * and write it back, to prevent weird side effects of modifying
+	 * an object file in place.
+	 */
+	fd_map = open(tmp_file, O_WRONLY | O_TRUNC | O_CREAT, sb.st_mode);
+	if (fd_map < 0) {
+		perror(fname);
+		fail_file();
+	}
+	n = write(fd_map, file_map, sb.st_size);
+	if (n != sb.st_size) {
+		perror("write");
+		fail_file();
+	}
+	if (file_append_size) {
+		n = write(fd_map, file_append, file_append_size);
+		if (n != file_append_size) {
+			perror("write");
 			fail_file();
 		}
-		uwrite(fd_map, addr, sb.st_size);
 	}
-	return addr;
+	close(fd_map);
+	if (rename(tmp_file, fname) < 0) {
+		perror(fname);
+		fail_file();
+	}
 }
 
 /* w8rev, w8nat, ...: Handle endianness. */
@@ -310,7 +382,6 @@ do_file(char const *const fname)
 	Elf32_Ehdr *const ehdr = mmap_file(fname);
 	unsigned int reltype = 0;
 
-	ehdr_curr = ehdr;
 	w = w4nat;
 	w2 = w2nat;
 	w8 = w8nat;
@@ -428,6 +499,7 @@ do_file(char const *const fname)
 	}
 	}  /* end switch */
 
+	write_file(fname);
 	cleanup();
 }
 
@@ -480,11 +552,14 @@ main(int argc, char *argv[])
 		case SJ_SETJMP:    /* normal sequence */
 			/* Avoid problems if early cleanup() */
 			fd_map = -1;
-			ehdr_curr = NULL;
 			mmap_failed = 1;
+			file_map = NULL;
+			file_ptr = NULL;
+			file_updated = 0;
 			do_file(file);
 			break;
 		case SJ_FAIL:    /* error in do_file or below */
+			sprintf("%s: failed\n", file);
 			++n_error;
 			break;
 		case SJ_SUCCEED:    /* premature success */