[PATCH] um: hostfs: catch EINTR and partial read/write
Benjamin Berg
benjamin at sipsolutions.net
Fri Jan 5 02:12:24 PST 2024
On Thu, 2024-01-04 at 23:27 +0100, Richard Weinberger wrote:
> On Fri, Nov 10, 2023 at 10:44 AM <benjamin at sipsolutions.net> wrote:
> >
> > From: Benjamin Berg <benjamin.berg at intel.com>
> >
> > The UM kernel uses signals for various purposes (SIGALRM for
> > scheduling
> > for example). These signals are interrupts for the UM kernel, which
> > should not affect file system operations from userspace processes.
> >
> > Said differently, in hostfs all operations are directly forwarded
> > to the
> > host and will block the entire UM kernel. As such, hostfs does not
> > permit other tasks to be scheduled while operations are ongoing and
> > these operations are fundamentally synchronous and not
> > interruptible.
> >
> > With all this, the only reasonable thing to do is to catch all
> > EINTR
> > situations and retry them. This includes retrying short read/write
> > operations in order to ensure they finish.
>
> I'm confused. Your patches makes hostfs robust to EINTR but it
> doesn't remove the blocking, right?
The intention was to really do all I/O blocking rather than letting us
be interrupted by signals.
For me, the motivation was consistency. However, without the change, a
userspace process that does not expect signals might behave
incorrectly. For me more important though was that a correctly behaving
userspace process that retries could also affect the reproducability of
runs when using time-travel mode.
> In what scenarios do you see hostfs failing due to signals right now?
Usually userspace will retry and there is no failure. But I could
imagine a simple userspace process that does not retry fail if
something else happens at the same time (SIGALRM or SIGIO for console
input).
I agree that refactoring OS helpers a bit does seem like a nicer
solution overall.
Benjamin
>
> > If, at any point, hostfs becomes able to push operations into the
> > background in order to schedule another task, then an abortion
> > mechanism
> > may be needed.
> >
> > Signed-off-by: Benjamin Berg <benjamin.berg at intel.com>
> > ---
> > arch/um/include/shared/os.h | 7 ++-
> > fs/hostfs/hostfs_user.c | 115 +++++++++++++++++++++++---------
> > ----
> > 2 files changed, 80 insertions(+), 42 deletions(-)
> >
> > diff --git a/arch/um/include/shared/os.h
> > b/arch/um/include/shared/os.h
> > index 0df646c6651e..7c5564ebc5bb 100644
> > --- a/arch/um/include/shared/os.h
> > +++ b/arch/um/include/shared/os.h
> > @@ -18,7 +18,12 @@
> > #include <sys/types.h>
> > #endif
> >
> > -#define CATCH_EINTR(expr) while ((errno = 0, ((expr) < 0)) &&
> > (errno == EINTR))
> > +#define CATCH_EINTR(expr)
> > ({ \
> > + int
> > __result; \
> > + while ((errno = 0, ((__result = (expr)) < 0))
> > && \
> > + (errno ==
> > EINTR)); \
> > +
> > __result; \
> > + })
>
> This needs to be a patch on it's own.
>
> > #define OS_TYPE_FILE 1
> > #define OS_TYPE_DIR 2
> > diff --git a/fs/hostfs/hostfs_user.c b/fs/hostfs/hostfs_user.c
> > index 840619e39a1a..dd30bcc6d58f 100644
> > --- a/fs/hostfs/hostfs_user.c
> > +++ b/fs/hostfs/hostfs_user.c
> > @@ -17,6 +17,7 @@
> > #include <sys/syscall.h>
> > #include "hostfs.h"
> > #include <utime.h>
> > +#include <os.h>
> >
> > static void stat64_to_hostfs(const struct stat64 *buf, struct
> > hostfs_stat *p)
> > {
> > @@ -44,9 +45,9 @@ int stat_file(const char *path, struct
> > hostfs_stat *p, int fd)
> > struct stat64 buf;
> >
> > if (fd >= 0) {
> > - if (fstat64(fd, &buf) < 0)
> > + if (CATCH_EINTR(fstat64(fd, &buf)) < 0)
> > return -errno;
> > - } else if (lstat64(path, &buf) < 0) {
> > + } else if (CATCH_EINTR(lstat64(path, &buf)) < 0) {
> > return -errno;
> > }
> > stat64_to_hostfs(&buf, p);
> > @@ -63,7 +64,7 @@ int access_file(char *path, int r, int w, int x)
> > mode |= W_OK;
> > if (x)
> > mode |= X_OK;
> > - if (access(path, mode) != 0)
> > + if (CATCH_EINTR(access(path, mode)) != 0)
> > return -errno;
> > else return 0;
> > }
> > @@ -82,7 +83,7 @@ int open_file(char *path, int r, int w, int
> > append)
> >
> > if (append)
> > mode |= O_APPEND;
> > - fd = open64(path, mode);
> > + fd = CATCH_EINTR(open64(path, mode));
> > if (fd < 0)
> > return -errno;
> > else return fd;
> > @@ -92,7 +93,11 @@ void *open_dir(char *path, int *err_out)
> > {
> > DIR *dir;
> >
> > - dir = opendir(path);
> > + do {
> > + errno = 0;
> > + dir = opendir(path);
> > + } while (dir == NULL && errno == -EINTR);
> > +
> > *err_out = errno;
> >
> > return dir;
> > @@ -112,9 +117,14 @@ char *read_dir(void *stream, unsigned long
> > long *pos_out,
> > DIR *dir = stream;
> > struct dirent *ent;
> >
> > - ent = readdir(dir);
> > + do {
> > + errno = 0;
> > + ent = readdir(dir);
> > + } while (ent == 0 && errno == EINTR);
> > +
> > if (ent == NULL)
> > return NULL;
> > +
> > *len_out = strlen(ent->d_name);
> > *ino_out = ent->d_ino;
> > *type_out = ent->d_type;
> > @@ -125,30 +135,52 @@ char *read_dir(void *stream, unsigned long
> > long *pos_out,
> > int read_file(int fd, unsigned long long *offset, char *buf, int
> > len)
> > {
> > int n;
> > + int read = 0;
> > +
> > + do {
> > + errno = 0;
> > + n = pread64(fd, buf, len, *offset);
> > + if (n > 0) {
> > + read += n;
> > + *offset += n;
> > + len -= n;
> > + continue;
> > + }
> > + } while (n < 0 && errno == EINTR);
> >
> > - n = pread64(fd, buf, len, *offset);
> > - if (n < 0)
> > - return -errno;
> > - *offset += n;
> > - return n;
> > + if (read > 0)
> > + return read;
> > +
> > + return -errno;
> > }
> >
> > int write_file(int fd, unsigned long long *offset, const char
> > *buf, int len)
> > {
> > int n;
> > + int written = 0;
> > +
> > + do {
> > + errno = 0;
> > + n = pwrite64(fd, buf, len, *offset);
> > + if (n > 0) {
> > + written += n;
> > + *offset += n;
> > + len -= n;
> > + continue;
> > + }
> > + } while (n < 0 && errno == EINTR);
> >
> > - n = pwrite64(fd, buf, len, *offset);
> > - if (n < 0)
> > - return -errno;
> > - *offset += n;
> > - return n;
> > + if (written > 0)
> > + return written;
> > +
> > + return -errno;
> > }
> >
> > int lseek_file(int fd, long long offset, int whence)
> > {
> > int ret;
> >
> > - ret = lseek64(fd, offset, whence);
> > + ret = CATCH_EINTR(lseek64(fd, offset, whence));
> > if (ret < 0)
> > return -errno;
> > return 0;
> > @@ -158,9 +190,9 @@ int fsync_file(int fd, int datasync)
> > {
> > int ret;
> > if (datasync)
> > - ret = fdatasync(fd);
> > + ret = CATCH_EINTR(fdatasync(fd));
> > else
> > - ret = fsync(fd);
> > + ret = CATCH_EINTR(fsync(fd));
> >
> > if (ret < 0)
> > return -errno;
> > @@ -169,24 +201,24 @@ int fsync_file(int fd, int datasync)
> >
> > int replace_file(int oldfd, int fd)
> > {
> > - return dup2(oldfd, fd);
> > + return CATCH_EINTR(dup2(oldfd, fd));
> > }
> >
> > void close_file(void *stream)
> > {
> > - close(*((int *) stream));
> > + CATCH_EINTR(close(*((int *) stream)));
> > }
> >
> > void close_dir(void *stream)
> > {
> > - closedir(stream);
> > + CATCH_EINTR(closedir(stream));
> > }
> >
> > int file_create(char *name, int mode)
> > {
> > int fd;
> >
> > - fd = open64(name, O_CREAT | O_RDWR, mode);
> > + fd = CATCH_EINTR(open64(name, O_CREAT | O_RDWR, mode));
> > if (fd < 0)
> > return -errno;
> > return fd;
> > @@ -200,33 +232,33 @@ int set_attr(const char *file, struct
> > hostfs_iattr *attrs, int fd)
> >
> > if (attrs->ia_valid & HOSTFS_ATTR_MODE) {
> > if (fd >= 0) {
> > - if (fchmod(fd, attrs->ia_mode) != 0)
> > + if (CATCH_EINTR(fchmod(fd, attrs->ia_mode))
> > != 0)
> > return -errno;
> > - } else if (chmod(file, attrs->ia_mode) != 0) {
> > + } else if (CATCH_EINTR(chmod(file, attrs->ia_mode))
> > != 0) {
> > return -errno;
> > }
> > }
> > if (attrs->ia_valid & HOSTFS_ATTR_UID) {
> > if (fd >= 0) {
> > - if (fchown(fd, attrs->ia_uid, -1))
> > + if (CATCH_EINTR(fchown(fd, attrs->ia_uid, -
> > 1)))
> > return -errno;
> > - } else if (chown(file, attrs->ia_uid, -1)) {
> > + } else if (CATCH_EINTR(chown(file, attrs->ia_uid, -
> > 1))) {
> > return -errno;
> > }
> > }
> > if (attrs->ia_valid & HOSTFS_ATTR_GID) {
> > if (fd >= 0) {
> > - if (fchown(fd, -1, attrs->ia_gid))
> > + if (CATCH_EINTR(fchown(fd, -1, attrs-
> > >ia_gid)))
> > return -errno;
> > - } else if (chown(file, -1, attrs->ia_gid)) {
> > + } else if (CATCH_EINTR(chown(file, -1, attrs-
> > >ia_gid))) {
> > return -errno;
> > }
> > }
> > if (attrs->ia_valid & HOSTFS_ATTR_SIZE) {
> > if (fd >= 0) {
> > - if (ftruncate(fd, attrs->ia_size))
> > + if (CATCH_EINTR(ftruncate(fd, attrs-
> > >ia_size)))
> > return -errno;
> > - } else if (truncate(file, attrs->ia_size)) {
> > + } else if (CATCH_EINTR(truncate(file, attrs-
> > >ia_size))) {
> > return -errno;
> > }
> > }
> > @@ -279,7 +311,7 @@ int make_symlink(const char *from, const char
> > *to)
> > {
> > int err;
> >
> > - err = symlink(to, from);
> > + err = CATCH_EINTR(symlink(to, from));
> > if (err)
> > return -errno;
> > return 0;
> > @@ -289,7 +321,7 @@ int unlink_file(const char *file)
> > {
> > int err;
> >
> > - err = unlink(file);
> > + err = CATCH_EINTR(unlink(file));
> > if (err)
> > return -errno;
> > return 0;
> > @@ -299,7 +331,7 @@ int do_mkdir(const char *file, int mode)
> > {
> > int err;
> >
> > - err = mkdir(file, mode);
> > + err = CATCH_EINTR(mkdir(file, mode));
> > if (err)
> > return -errno;
> > return 0;
> > @@ -309,7 +341,7 @@ int hostfs_do_rmdir(const char *file)
> > {
> > int err;
> >
> > - err = rmdir(file);
> > + err = CATCH_EINTR(rmdir(file));
> > if (err)
> > return -errno;
> > return 0;
> > @@ -319,7 +351,7 @@ int do_mknod(const char *file, int mode,
> > unsigned int major, unsigned int minor)
> > {
> > int err;
> >
> > - err = mknod(file, mode, os_makedev(major, minor));
> > + err = CATCH_EINTR(mknod(file, mode, os_makedev(major,
> > minor)));
> > if (err)
> > return -errno;
> > return 0;
> > @@ -329,7 +361,7 @@ int link_file(const char *to, const char *from)
> > {
> > int err;
> >
> > - err = link(to, from);
> > + err = CATCH_EINTR(link(to, from));
> > if (err)
> > return -errno;
> > return 0;
> > @@ -339,7 +371,7 @@ int hostfs_do_readlink(char *file, char *buf,
> > int size)
> > {
> > int n;
> >
> > - n = readlink(file, buf, size);
> > + n = CATCH_EINTR(readlink(file, buf, size));
> > if (n < 0)
> > return -errno;
> > if (n < size)
> > @@ -351,7 +383,7 @@ int rename_file(char *from, char *to)
> > {
> > int err;
> >
> > - err = rename(from, to);
> > + err = CATCH_EINTR(rename(from, to));
> > if (err < 0)
> > return -errno;
> > return 0;
> > @@ -371,7 +403,8 @@ int rename2_file(char *from, char *to, unsigned
> > int flags)
> > #endif
> >
> > #ifdef SYS_renameat2
> > - err = syscall(SYS_renameat2, AT_FDCWD, from, AT_FDCWD, to,
> > flags);
> > + err = CATCH_EINTR(syscall(SYS_renameat2, AT_FDCWD, from,
> > + AT_FDCWD, to, flags));
> > if (err < 0) {
> > if (errno != ENOSYS)
> > return -errno;
> > @@ -392,7 +425,7 @@ int do_statfs(char *root, long *bsize_out, long
> > long *blocks_out,
> > struct statfs64 buf;
> > int err;
> >
> > - err = statfs64(root, &buf);
> > + err = CATCH_EINTR(statfs64(root, &buf));
> > if (err < 0)
> > return -errno;
>
> In general I'm not so happy with adding CATCH_EINTR() to all call
> sites.
> As Anton suggested, a new helper would be nice for each file method.
>
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