[PATCH] bcachefs: ioctl: avoid stack overflow warning

[PATCH] bcachefs: ioctl: avoid stack overflow warning

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.de>

Multiple ioctl handlers individually use a lot of stack space, and clang chooses
to inline them into the bch2_fs_ioctl() function, blowing through the warning
limit:

fs/bcachefs/chardev.c:655:6: error: stack frame size (1032) exceeds limit (1024) in 'bch2_fs_ioctl' [-Werror,-Wframe-larger-than]
  655 | long bch2_fs_ioctl(struct bch_fs *c, unsigned cmd, void __user *arg)

By marking the largest two of them as noinline_for_stack, no indidual code path
ends up using this much, which avoids the warning and reduces the possible
total stack usage in the ioctl handler.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 fs/bcachefs/chardev.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/bcachefs/chardev.c b/fs/bcachefs/chardev.c
index 2d38466eddfd..fde3c2380e28 100644
--- a/fs/bcachefs/chardev.c
+++ b/fs/bcachefs/chardev.c
@@ -399,7 +399,7 @@ static long bch2_ioctl_data(struct bch_fs *c,
 	return ret;
 }
 
-static long bch2_ioctl_fs_usage(struct bch_fs *c,
+static noinline_for_stack long bch2_ioctl_fs_usage(struct bch_fs *c,
 				struct bch_ioctl_fs_usage __user *user_arg)
 {
 	struct bch_ioctl_fs_usage arg = {};
@@ -469,7 +469,7 @@ static long bch2_ioctl_query_accounting(struct bch_fs *c,
 }
 
 /* obsolete, didn't allow for new data types: */
-static long bch2_ioctl_dev_usage(struct bch_fs *c,
+static noinline_for_stack long bch2_ioctl_dev_usage(struct bch_fs *c,
 				 struct bch_ioctl_dev_usage __user *user_arg)
 {
 	struct bch_ioctl_dev_usage arg;
-- 
2.39.5

Re: [PATCH] bcachefs: ioctl: avoid stack overflow warning

[Kent Overstreet]

On Tue, Jun 10, 2025 at 11:24:04AM +0200, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> Multiple ioctl handlers individually use a lot of stack space, and clang chooses
> to inline them into the bch2_fs_ioctl() function, blowing through the warning
> limit:
> 
> fs/bcachefs/chardev.c:655:6: error: stack frame size (1032) exceeds limit (1024) in 'bch2_fs_ioctl' [-Werror,-Wframe-larger-than]
>   655 | long bch2_fs_ioctl(struct bch_fs *c, unsigned cmd, void __user *arg)
> 
> By marking the largest two of them as noinline_for_stack, no indidual code path
> ends up using this much, which avoids the warning and reduces the possible
> total stack usage in the ioctl handler.
> 
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Applied, thanks

> ---
>  fs/bcachefs/chardev.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/bcachefs/chardev.c b/fs/bcachefs/chardev.c
> index 2d38466eddfd..fde3c2380e28 100644
> --- a/fs/bcachefs/chardev.c
> +++ b/fs/bcachefs/chardev.c
> @@ -399,7 +399,7 @@ static long bch2_ioctl_data(struct bch_fs *c,
>  	return ret;
>  }
>  
> -static long bch2_ioctl_fs_usage(struct bch_fs *c,
> +static noinline_for_stack long bch2_ioctl_fs_usage(struct bch_fs *c,
>  				struct bch_ioctl_fs_usage __user *user_arg)
>  {
>  	struct bch_ioctl_fs_usage arg = {};
> @@ -469,7 +469,7 @@ static long bch2_ioctl_query_accounting(struct bch_fs *c,
>  }
>  
>  /* obsolete, didn't allow for new data types: */
> -static long bch2_ioctl_dev_usage(struct bch_fs *c,
> +static noinline_for_stack long bch2_ioctl_dev_usage(struct bch_fs *c,
>  				 struct bch_ioctl_dev_usage __user *user_arg)
>  {
>  	struct bch_ioctl_dev_usage arg;
> -- 
> 2.39.5
>