Userspace generally expects APIs that return EMSGSIZE to allow for them to adjust their buffer size and retry the operation. However, the fscontext log would previously clear the message even in the EMSGSIZE case.
Given that it is very cheap for us to check whether the buffer is too small before we remove the message from the ring buffer, let's just do that instead.
Fixes: 007ec26cdc9f ("vfs: Implement logging through fs_context") Signed-off-by: Aleksa Sarai cyphar@cyphar.com --- Aleksa Sarai (2): fscontext: do not consume log entries for -EMSGSIZE case selftests/filesystems: add basic fscontext log tests
fs/fsopen.c | 22 ++-- tools/testing/selftests/filesystems/.gitignore | 1 + tools/testing/selftests/filesystems/Makefile | 2 +- tools/testing/selftests/filesystems/fclog.c | 137 +++++++++++++++++++++++++ 4 files changed, 153 insertions(+), 9 deletions(-) --- base-commit: 66639db858112bf6b0f76677f7517643d586e575 change-id: 20250806-fscontext-log-cleanups-50f0143674ae
Best regards,
Userspace generally expects APIs that return EMSGSIZE to allow for them to adjust their buffer size and retry the operation. However, the fscontext log would previously clear the message even in the EMSGSIZE case.
Given that it is very cheap for us to check whether the buffer is too small before we remove the message from the ring buffer, let's just do that instead.
Fixes: 007ec26cdc9f ("vfs: Implement logging through fs_context") Cc: David Howells dhowells@redhat.com Cc: stable@vger.kernel.org # v5.2+ Signed-off-by: Aleksa Sarai cyphar@cyphar.com --- fs/fsopen.c | 22 ++++++++++++++-------- 1 file changed, 14 insertions(+), 8 deletions(-)
diff --git a/fs/fsopen.c b/fs/fsopen.c index 1aaf4cb2afb2..f5fdaa97965b 100644 --- a/fs/fsopen.c +++ b/fs/fsopen.c @@ -36,23 +36,25 @@ static ssize_t fscontext_read(struct file *file, if (ret < 0) return ret;
- if (log->head == log->tail) { - mutex_unlock(&fc->uapi_mutex); - return -ENODATA; - } + ret = -ENODATA; + if (log->head == log->tail) + goto err_unlock_nomsg;
index = log->tail & (logsize - 1); p = log->buffer[index]; + n = strlen(p); + + ret = -EMSGSIZE; + if (n > len) + goto err_unlock_nomsg; + + /* Consume the message from the queue. */ need_free = log->need_free & (1 << index); log->buffer[index] = NULL; log->need_free &= ~(1 << index); log->tail++; mutex_unlock(&fc->uapi_mutex);
- ret = -EMSGSIZE; - n = strlen(p); - if (n > len) - goto err_free; ret = -EFAULT; if (copy_to_user(_buf, p, n) != 0) goto err_free; @@ -62,6 +64,10 @@ static ssize_t fscontext_read(struct file *file, if (need_free) kfree(p); return ret; + +err_unlock_nomsg: + mutex_unlock(&fc->uapi_mutex); + return ret; }
static int fscontext_release(struct inode *inode, struct file *file)
On Wed, Aug 06, 2025 at 03:31:10PM +1000, Aleksa Sarai wrote:
Userspace generally expects APIs that return EMSGSIZE to allow for them to adjust their buffer size and retry the operation. However, the fscontext log would previously clear the message even in the EMSGSIZE case.
Given that it is very cheap for us to check whether the buffer is too small before we remove the message from the ring buffer, let's just do that instead.
Fixes: 007ec26cdc9f ("vfs: Implement logging through fs_context") Cc: David Howells dhowells@redhat.com Cc: stable@vger.kernel.org # v5.2+ Signed-off-by: Aleksa Sarai cyphar@cyphar.com
fs/fsopen.c | 22 ++++++++++++++-------- 1 file changed, 14 insertions(+), 8 deletions(-)
diff --git a/fs/fsopen.c b/fs/fsopen.c index 1aaf4cb2afb2..f5fdaa97965b 100644 --- a/fs/fsopen.c +++ b/fs/fsopen.c @@ -36,23 +36,25 @@ static ssize_t fscontext_read(struct file *file, if (ret < 0) return ret;
- if (log->head == log->tail) {
mutex_unlock(&fc->uapi_mutex);return -ENODATA;- }
- ret = -ENODATA;
- if (log->head == log->tail)
goto err_unlock_nomsg;index = log->tail & (logsize - 1); p = log->buffer[index];
- n = strlen(p);
- ret = -EMSGSIZE;
- if (n > len)
goto err_unlock_nomsg;
FWIW, I would rather turn that into a helper taking log and len and returning p or ERR_PTR(...); something like
static inline const char *fetch_message(struct fc_log *log, size_t len, bool *need_free) { int index = log->tail & (ARRAY_SIZE(log->buffer) - 1); const char *p = log->buffer[index];
if (unlikely(log->head == log->tail)) return ERR_PTR(-ENODATA);
n = strlen(p); if (unlikely(n > len)) return ERR_PTR(-EMSGSIZE);
log->buffer[index] = NULL; *need_free = log->need_free & (1 << index); log->need_free &= ~(1 << index); log->tail++;
return p; }
with caller being
ret = mutex_lock_interruptible(&fc->uapi_mutex); if (ret < 0) return ret; p = fetch_message(log, len, &need_free); mutex_unlock(&fc->uapi_mutex); if (IS_ERR(p)) return PTR_ERR(p); n = strlen(p); if (copy_to_user(_buf, p, n)) n = -EFAULT; if (need_free) kfree(p); return n;
and that's it.
linux-stable-mirror@lists.linaro.org
-
Al Viro -
Aleksa Sarai