Subject: Re: [PATCH 2/7] fsmonitor: determine if filesystem is local or remote

From: Patrick Steinhardt <[email-masked]>
Date: Thu, 15 Feb 2024 14:49:27 +0100

On Thu, Feb 15, 2024 at 10:29:33AM +0000, Eric DeCosta via GitGitGadget wrote:
> From: Eric DeCosta <[email-masked]>
> 
> Compare the given path to the mounted filesystems. Find the mount that is
> the longest prefix of the path (if any) and determine if that mount is on a
> local or remote filesystem.

It would be nice to motivate this change in the commit message. Right
now it only explains what the commit does, but it does not mention at
all why that would be a good idea in the first place. Explaining that
this is part of the interface that the existing fsmonitor infrastructure
expects to exist would help.

> Signed-off-by: Eric DeCosta <[email-masked]>
> ---
>  Makefile                                |   4 +
>  compat/fsmonitor/fsm-path-utils-linux.c | 195 ++++++++++++++++++++++++
>  compat/fsmonitor/fsm-path-utils-linux.h |  91 +++++++++++
>  config.mak.uname                        |  11 ++
>  4 files changed, 301 insertions(+)
>  create mode 100644 compat/fsmonitor/fsm-path-utils-linux.c
>  create mode 100644 compat/fsmonitor/fsm-path-utils-linux.h
> 
> diff --git a/Makefile b/Makefile
> index 78e874099d9..0f36a0fd83a 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -2088,6 +2088,10 @@ ifdef HAVE_CLOCK_GETTIME
>  	BASIC_CFLAGS += -DHAVE_CLOCK_GETTIME
>  endif
>  
> +ifdef HAVE_LINUX_MAGIC_H
> +	BASIC_CFLAGS += -DHAVE_LINUX_MAGIC_H
> +endif
> +
>  ifdef HAVE_CLOCK_MONOTONIC
>  	BASIC_CFLAGS += -DHAVE_CLOCK_MONOTONIC
>  endif
> diff --git a/compat/fsmonitor/fsm-path-utils-linux.c b/compat/fsmonitor/fsm-path-utils-linux.c
> new file mode 100644
> index 00000000000..c21d1349532
> --- /dev/null
> +++ b/compat/fsmonitor/fsm-path-utils-linux.c
> @@ -0,0 +1,195 @@
> +#include "git-compat-util.h"
> +#include "abspath.h"
> +#include "fsmonitor.h"
> +#include "fsmonitor-path-utils.h"
> +#include "fsm-path-utils-linux.h"
> +#include <errno.h>
> +#include <mntent.h>
> +#include <sys/mount.h>
> +#include <sys/vfs.h>
> +#include <sys/statvfs.h>
> +
> +static int is_remote_fs(const char *path)
> +{
> +	struct statfs fs;
> +
> +	if (statfs(path, &fs))
> +		return error_errno(_("statfs('%s') failed"), path);
> +
> +	switch (fs.f_type) {
> +	case ACFS_SUPER_MAGIC:
> +	case AFS_SUPER_MAGIC:
> +	case CEPH_SUPER_MAGIC:
> +	case CIFS_SUPER_MAGIC:
> +	case CODA_SUPER_MAGIC:
> +	case FHGFS_SUPER_MAGIC:
> +	case GFS_SUPER_MAGIC:
> +	case GPFS_SUPER_MAGIC:
> +	case IBRIX_SUPER_MAGIC:
> +	case KAFS_SUPER_MAGIC:
> +	case LUSTRE_SUPER_MAGIC:
> +	case NCP_SUPER_MAGIC:
> +	case NFS_SUPER_MAGIC:
> +	case NFSD_SUPER_MAGIC:
> +	case OCFS2_SUPER_MAGIC:
> +	case PANFS_SUPER_MAGIC:
> +	case SMB_SUPER_MAGIC:
> +	case SMB2_SUPER_MAGIC:
> +	case SNFS_SUPER_MAGIC:
> +	case VMHGFS_SUPER_MAGIC:
> +	case VXFS_SUPER_MAGIC:
> +		return 1;
> +	default:
> +		return 0;
> +	}
> +}

This list doesn't feel all that maintainable to me, but so be it if
there is no better interface available.

> +static int find_mount(const char *path, const struct statvfs *fs,
> +			struct mntent *entry)

I don't quite understand why `find_mount()` is required in the first
place. Why can't we statfs(2) the path directly? This syscall provides
the fsid and should be sufficient for us to fill in `struct fs_info`.

Explaining details like this in the commit message would help guide the
reader's expectations.

Patrick

> +{
> +	const char *const mounts = "/proc/mounts";
> +	char *rp = real_pathdup(path, 1);
> +	struct mntent *ment = NULL;
> +	struct statvfs mntfs;
> +	FILE *fp;
> +	int found = 0;
> +	int ret = 0;
> +	size_t dlen, plen, flen = 0;
> +
> +	entry->mnt_fsname = NULL;
> +	entry->mnt_dir = NULL;
> +	entry->mnt_type = NULL;
> +
> +	fp = setmntent(mounts, "r");
> 
> +	if (!fp) {
> +		free(rp);
> +		return error_errno(_("setmntent('%s') failed"), mounts);
> +	}
> +
> +	plen = strlen(rp);
> +
> +	/* read all the mount information and compare to path */
> +	while ((ment = getmntent(fp))) {
> +		if (statvfs(ment->mnt_dir, &mntfs)) {
> +			switch (errno) {
> +			case EPERM:
> +			case ESRCH:
> +			case EACCES:
> +				continue;
> +			default:
> +				error_errno(_("statvfs('%s') failed"), ment->mnt_dir);
> +				ret = -1;
> +				goto done;
> +			}
> +		}
> +
> +		/* is mount on the same filesystem and is a prefix of the path */
> +		if ((fs->f_fsid == mntfs.f_fsid) &&
> +			!strncmp(ment->mnt_dir, rp, strlen(ment->mnt_dir))) {
> +			dlen = strlen(ment->mnt_dir);
> +			if (dlen > plen)
> +				continue;
> +			/*
> +			 * look for the longest prefix (including root)
> +			 */
> +			if (dlen > flen &&
> +				((dlen == 1 && ment->mnt_dir[0] == '/') ||
> +				 (!rp[dlen] || rp[dlen] == '/'))) {
> +				flen = dlen;
> +				found = 1;
> +
> +				/*
> +				 * https://man7.org/linux/man-pages/man3/getmntent.3.html
> +				 *
> +				 * The pointer points to a static area of memory which is
> +				 * overwritten by subsequent calls to getmntent().
> +				 */
> +				free(entry->mnt_fsname);
> +				free(entry->mnt_dir);
> +				free(entry->mnt_type);
> +				entry->mnt_fsname = xstrdup(ment->mnt_fsname);
> +				entry->mnt_dir = xstrdup(ment->mnt_dir);
> +				entry->mnt_type = xstrdup(ment->mnt_type);
> +			}
> +		}
> +	}
> +
> +done:
> +	free(rp);
> +	endmntent(fp);
> +
> +	if (!found)
> +		return -1;
> +
> +	return ret;
> +}
> +
> +int fsmonitor__get_fs_info(const char *path, struct fs_info *fs_info)
> +{
> +	int ret = 0;
> +	struct mntent entry;
> +	struct statvfs fs;
> +
> +	fs_info->is_remote = -1;
> +	fs_info->typename = NULL;
> +
> +	if (statvfs(path, &fs))
> +		return error_errno(_("statvfs('%s') failed"), path);
> +
> +	if (find_mount(path, &fs, &entry) < 0) {
> +		ret = -1;
> +		goto done;
> +	}
> +
> +	trace_printf_key(&trace_fsmonitor,
> +			 "statvfs('%s') [flags 0x%08lx] '%s' '%s'",
> +			 path, fs.f_flag, entry.mnt_type, entry.mnt_fsname);
> +
> +	fs_info->is_remote = is_remote_fs(entry.mnt_dir);
> +	fs_info->typename = xstrdup(entry.mnt_fsname);
> +
> +	if (fs_info->is_remote < 0)
> +		ret = -1;
> +
> +	trace_printf_key(&trace_fsmonitor,
> +				"'%s' is_remote: %d",
> +				path, fs_info->is_remote);
> +
> +done:
> +	free(entry.mnt_fsname);
> +	free(entry.mnt_dir);
> +	free(entry.mnt_type);
> +	return ret;
> +}
> +
> +int fsmonitor__is_fs_remote(const char *path)
> +{
> +	int ret = 0;
> +	struct fs_info fs;
> +
> +	if (fsmonitor__get_fs_info(path, &fs))
> +		ret = -1;
> +	else
> +		ret = fs.is_remote;
> +
> +	free(fs.typename);
> +
> +	return ret;
> +}
> +
> +/*
> + * No-op for now.
> + */
> +int fsmonitor__get_alias(const char *path, struct alias_info *info)
> +{
> +	return 0;
> +}
> +
> +/*
> + * No-op for now.
> + */
> +char *fsmonitor__resolve_alias(const char *path,
> +		const struct alias_info *info)
> +{
> +	return NULL;
> +}
> diff --git a/compat/fsmonitor/fsm-path-utils-linux.h b/compat/fsmonitor/fsm-path-utils-linux.h
> new file mode 100644
> index 00000000000..49bdb3c4728
> --- /dev/null
> +++ b/compat/fsmonitor/fsm-path-utils-linux.h
> @@ -0,0 +1,91 @@
> +#ifndef FSM_PATH_UTILS_LINUX_H
> +#define FSM_PATH_UTILS_LINUX_H
> +#endif
> +
> +#ifdef HAVE_LINUX_MAGIC_H
> +#include <linux/magic.h>
> +#endif
> +
> +#ifndef ACFS_SUPER_MAGIC
> +#define ACFS_SUPER_MAGIC 0x61636673
> +#endif
> +
> +#ifndef AFS_SUPER_MAGIC
> +#define AFS_SUPER_MAGIC 0x5346414f
> +#endif
> +
> +#ifndef CEPH_SUPER_MAGIC
> +#define CEPH_SUPER_MAGIC 0x00c36400
> +#endif
> +
> +#ifndef CIFS_SUPER_MAGIC
> +#define CIFS_SUPER_MAGIC 0xff534d42
> +#endif
> +
> +#ifndef CODA_SUPER_MAGIC
> +#define CODA_SUPER_MAGIC 0x73757245
> +#endif
> +
> +#ifndef FHGFS_SUPER_MAGIC
> +#define FHGFS_SUPER_MAGIC 0x19830326
> +#endif
> +
> +#ifndef GFS_SUPER_MAGIC
> +#define GFS_SUPER_MAGIC 0x1161970
> +#endif
> +
> +#ifndef GPFS_SUPER_MAGIC
> +#define GPFS_SUPER_MAGIC 0x47504653
> +#endif
> +
> +#ifndef IBRIX_SUPER_MAGIC
> +#define IBRIX_SUPER_MAGIC 0x013111a8
> +#endif
> +
> +#ifndef KAFS_SUPER_MAGIC
> +#define KAFS_SUPER_MAGIC 0x6b414653
> +#endif
> +
> +#ifndef LUSTRE_SUPER_MAGIC
> +#define LUSTRE_SUPER_MAGIC 0x0bd00bd0
> +#endif
> +
> +#ifndef NCP_SUPER_MAGIC
> +#define NCP_SUPER_MAGIC 0x564c
> +#endif
> +
> +#ifndef NFS_SUPER_MAGIC
> +#define NFS_SUPER_MAGIC 0x6969
> +#endif
> +
> +#ifndef NFSD_SUPER_MAGIC
> +#define NFSD_SUPER_MAGIC 0x6e667364
> +#endif
> +
> +#ifndef OCFS2_SUPER_MAGIC
> +#define OCFS2_SUPER_MAGIC 0x7461636f
> +#endif
> +
> +#ifndef PANFS_SUPER_MAGIC
> +#define PANFS_SUPER_MAGIC 0xaad7aaea
> +#endif
> +
> +#ifndef SMB_SUPER_MAGIC
> +#define SMB_SUPER_MAGIC 0x517b
> +#endif
> +
> +#ifndef SMB2_SUPER_MAGIC
> +#define SMB2_SUPER_MAGIC 0xfe534d42
> +#endif
> +
> +#ifndef SNFS_SUPER_MAGIC
> +#define SNFS_SUPER_MAGIC 0xbeefdead
> +#endif
> +
> +#ifndef VMHGFS_SUPER_MAGIC
> +#define VMHGFS_SUPER_MAGIC 0xbacbacbc
> +#endif
> +
> +#ifndef VXFS_SUPER_MAGIC
> +#define VXFS_SUPER_MAGIC 0xa501fcf5
> +#endif
> diff --git a/config.mak.uname b/config.mak.uname
> index dacc95172dc..80d7e2a2e68 100644
> --- a/config.mak.uname
> +++ b/config.mak.uname
> @@ -68,6 +68,17 @@ ifeq ($(uname_S),Linux)
>  	ifneq ($(findstring .el7.,$(uname_R)),)
>  		BASIC_CFLAGS += -std=c99
>  	endif
> +	ifeq ($(shell test -f /usr/include/linux/magic.h && echo y),y)
> +		HAVE_LINUX_MAGIC_H = YesPlease
> +	endif
> +	# The builtin FSMonitor on Linux builds upon Simple-IPC.  Both require
> +	# Unix domain sockets and PThreads.
> +	ifndef NO_PTHREADS
> +	ifndef NO_UNIX_SOCKETS
> +	FSMONITOR_DAEMON_BACKEND = linux
> +	FSMONITOR_OS_SETTINGS = linux
> +	endif
> +	endif
>  endif
>  ifeq ($(uname_S),GNU/kFreeBSD)
>  	HAVE_ALLOCA_H = YesPlease
> -- 
> gitgitgadget
> 
>