erofs-utils: dump: Add --cat flag to show file contents

This patch adds a new '--cat' flag to the 'dump.erofs' command. When
used it will write to the standard output the content of the file
indicated by the '--path' or '--nid' options. For example, if there is a
'/mydir/myfile.txt' file containg the text 'mytext':

    $ dump.erofs --cat --path=/mydir/myfile.txt myimage.erofs
    mytext

Signed-off-by: Juan Hernandez <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Gao Xiang <[email protected]>

dump/main.c

@@ -26,6 +26,7 @@ struct erofsdump_cfg {
 	bool show_superblock;
 	bool show_statistics;
 	bool show_subdirectories;
+	bool show_file_content;
 	erofs_nid_t nid;
 	const char *inode_path;
 };
@@ -80,6 +81,7 @@ static struct option long_options[] = {
 	{"path", required_argument, NULL, 4},
 	{"ls", no_argument, NULL, 5},
 	{"offset", required_argument, NULL, 6},
+	{"cat", no_argument, NULL, 7},
 	{0, 0, 0, 0},
 };
 
@@ -123,6 +125,7 @@ static void usage(int argc, char **argv)
 		" -s              show information about superblock\n"
 		" --device=X      specify an extra device to be used together\n"
 		" --ls            show directory contents (INODE required)\n"
+		" --cat           show file contents (INODE required)\n"
 		" --nid=#         show the target inode info of nid #\n"
 		" --offset=#      skip # bytes at the beginning of IMAGE\n"
 		" --path=X        show the target inode info of path X\n",
@@ -186,6 +189,9 @@ static int erofsdump_parse_options_cfg(int argc, char **argv)
 				return -EINVAL;
 			}
 			break;
+		case 7:
+			dumpcfg.show_file_content = true;
+			break;
 		default:
 			return -EINVAL;
 		}
@@ -672,6 +678,56 @@ static void erofsdump_show_superblock(void)
 			uuid_str);
 }
 
+static void erofsdump_show_file_content(void)
+{
+	int err;
+	struct erofs_inode inode = { .sbi = &g_sbi, .nid = dumpcfg.nid };
+	size_t buffer_size;
+	char *buffer_ptr;
+	erofs_off_t pending_size;
+	erofs_off_t read_offset;
+	erofs_off_t read_size;
+
+	if (dumpcfg.inode_path) {
+		err = erofs_ilookup(dumpcfg.inode_path, &inode);
+		if (err) {
+			erofs_err("read inode failed @ %s", dumpcfg.inode_path);
+			return;
+		}
+	} else {
+		err = erofs_read_inode_from_disk(&inode);
+		if (err) {
+			erofs_err("read inode failed @ nid %llu", inode.nid | 0ULL);
+			return;
+		}
+	}
+
+	buffer_size = erofs_blksiz(inode.sbi);
+	buffer_ptr = malloc(buffer_size);
+	if (!buffer_ptr) {
+		erofs_err("buffer allocation failed @ nid %llu", inode.nid | 0ULL);
+		return;
+	}
+
+	pending_size = inode.i_size;
+	read_offset = 0;
+	while (pending_size > 0) {
+		read_size = pending_size > buffer_size? buffer_size: pending_size;
+		err = erofs_pread(&inode, buffer_ptr, read_size, read_offset);
+		if (err) {
+			erofs_err("read file failed @ nid %llu", inode.nid | 0ULL);
+			goto out;
+		}
+		pending_size -= read_size;
+		read_offset += read_size;
+		fwrite(buffer_ptr, read_size, 1, stdout);
+	}
+	fflush(stdout);
+
+out:
+	free(buffer_ptr);
+}
+
 int main(int argc, char **argv)
 {
 	int err;
@@ -696,6 +752,15 @@ int main(int argc, char **argv)
 		goto exit_dev_close;
 	}
 
+	if (dumpcfg.show_file_content) {
+		if (dumpcfg.show_superblock || dumpcfg.show_statistics || dumpcfg.show_subdirectories) {
+			fprintf(stderr, "The '--cat' flag is incompatible with '-S', '-e', '-s' and '--ls'.\n");
+			goto exit_dev_close;
+		}
+		erofsdump_show_file_content();
+		goto exit_dev_close;
+	}
+
 	if (!dumpcfg.totalshow) {
 		dumpcfg.show_superblock = true;
 		dumpcfg.totalshow = 1;

man/dump.erofs.1

@@ -8,15 +8,17 @@ or overall disk statistics information from an EROFS-formatted image.
 \fBdump.erofs\fR [\fIOPTIONS\fR] \fIIMAGE\fR
 .SH DESCRIPTION
 .B dump.erofs
-is used to retrieve erofs metadata from \fIIMAGE\fP and demonstrate
+is used to retrieve erofs metadata and data from \fIIMAGE\fP and demonstrate
 .br
 1) overall disk statistics,
 .br
 2) superblock information,
 .br
 3) file information of the given inode NID,
 .br
-4) file extent information of the given inode NID.
+4) file extent information of the given inode NID,
+.br
+5) file content for the given inode NID.
 .SH OPTIONS
 .TP
 .BI "\-\-device=" path
@@ -32,6 +34,13 @@ or
 .I path
 required.
 .TP
+.BI "\-\-cat"
+Write file content to standard output.
+.I NID
+or
+.I path
+required.
+.TP
 .BI "\-\-nid=" NID
 Specify an inode NID in order to print its file information.
 .TP