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the nix store may contain hardlinks: derivations may output them directly, or users may be using store optimization which automatically hardlinks identical files in the nix store. The presence of these links are intended to be a 'transparent' optimization. However, when creating a squashfs image, the image will be different depending on whether hard links were present on the filesystem, leading to reproducibility problems. By passing '-no-hardlinks' to mksquashfs the files are stored as duplicates in the squashfs image. Since squashfs has support for duplicate files this does not lead to a larger image. For more details see https://github.com/NixOS/nixpkgs/issues/114331
76 lines
3 KiB
Diff
76 lines
3 KiB
Diff
From d925c9a11ee2e88ac8aac03f51892746f2bcf8cd Mon Sep 17 00:00:00 2001
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From: Phillip Lougher <phillip@squashfs.org.uk>
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Date: Thu, 25 Feb 2021 23:12:10 +0000
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Subject: [PATCH] Mksquashfs: add -no-hardlinks option
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Normally Mksquashfs will detect hardlinks (multiple files with the
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same inode) and hardlink them in the Squashfs image.
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But often hardlinks are used in the original filesystem
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to save space, when files are discovered to be duplicate.
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In this special case the only reason the files are
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hardlinked is to save space, and where the filesystem
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doesn't handle duplicate files (different inode, same
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data).
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Squashfs does handle duplicate files, and so add
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an option to ignore hardlinks and instead
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store them as duplicates.
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Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
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---
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squashfs-tools/mksquashfs.c | 16 +++++++++++-----
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1 file changed, 11 insertions(+), 5 deletions(-)
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diff --git a/squashfs-tools/mksquashfs.c b/squashfs-tools/mksquashfs.c
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index a45b77f..d4dc359 100644
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--- a/squashfs-tools/mksquashfs.c
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+++ b/squashfs-tools/mksquashfs.c
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@@ -312,6 +312,9 @@ struct dir_info *root_dir;
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FILE *log_fd;
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int logging=FALSE;
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+/* Should Mksquashfs detect hardlinked files? */
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+int no_hardlinks = FALSE;
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+
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static char *read_from_disk(long long start, unsigned int avail_bytes);
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void add_old_root_entry(char *name, squashfs_inode inode, int inode_number,
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int type);
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@@ -3093,11 +3096,11 @@ struct inode_info *lookup_inode3(struct stat *buf, int pseudo, int id,
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/*
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* Look-up inode in hash table, if it already exists we have a
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- * hard-link, so increment the nlink count and return it.
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- * Don't do the look-up for directories because we don't hard-link
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- * directories.
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+ * hardlink, so increment the nlink count and return it.
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+ * Don't do the look-up for directories because Unix/Linux doesn't
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+ * allow hard-links to directories.
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*/
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- if ((buf->st_mode & S_IFMT) != S_IFDIR) {
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+ if ((buf->st_mode & S_IFMT) != S_IFDIR && !no_hardlinks) {
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for(inode = inode_info[ino_hash]; inode; inode = inode->next) {
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if(memcmp(buf, &inode->buf, sizeof(struct stat)) == 0) {
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inode->nlink ++;
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@@ -5447,7 +5450,9 @@ int main(int argc, char *argv[])
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comp = lookup_compressor(COMP_DEFAULT);
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for(i = source + 2; i < argc; i++) {
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- if(strcmp(argv[i], "-mkfs-time") == 0 ||
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+ if(strcmp(argv[i], "-no-hardlinks") == 0)
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+ no_hardlinks = TRUE;
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+ else if(strcmp(argv[i], "-mkfs-time") == 0 ||
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strcmp(argv[i], "-fstime") == 0) {
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if((++i == argc) || !parse_num_unsigned(argv[i], &mkfs_time)) {
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ERROR("%s: %s missing or invalid time value\n", argv[0], argv[i - 1]);
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@@ -5893,6 +5898,7 @@ printOptions:
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"files larger than block size\n");
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ERROR("-no-duplicates\t\tdo not perform duplicate "
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"checking\n");
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+ ERROR("-no-hardlinks\t\tdo not hardlink files, instead store duplicates\n");
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ERROR("-all-root\t\tmake all files owned by root\n");
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ERROR("-root-mode <mode>\tset root directory permissions to octal <mode>\n");
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ERROR("-force-uid <uid>\tset all file uids to <uid>\n");
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--
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2.30.0
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