Re: [PATCH 03/33] fuse: make debugging configurable at runtime

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, May 13, 2026 at 07:46:05PM +0200, Bernd Schubert wrote:
> 
> 
> On 5/13/26 19:41, Darrick J. Wong wrote:
> > On Wed, Apr 29, 2026 at 07:24:28AM -0700, Darrick J. Wong wrote:
> >> From: Darrick J. Wong <djwong@kernel.org>
> >>
> >> Use static keys so that we can configure debugging assertions and dmesg
> >> warnings at runtime.  By default this is turned off so the cost is
> >> merely scanning a nop sled.  However, fuse server developers can turn
> >> it on for their debugging systems.
> >>
> >> Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
> >> ---
> >>  fs/fuse/fuse_iomap_i.h |   17 ++++++++++++++---
> >>  fs/fuse/Kconfig        |   15 +++++++++++++++
> >>  fs/fuse/fuse_iomap.c   |   43 +++++++++++++++++++++++++++++++++++++++++++
> >>  3 files changed, 72 insertions(+), 3 deletions(-)
> >>
> >>
> >> diff --git a/fs/fuse/fuse_iomap_i.h b/fs/fuse/fuse_iomap_i.h
> >> index b9ab8ce140e8e1..c37a7c5cfc862f 100644
> >> --- a/fs/fuse/fuse_iomap_i.h
> >> +++ b/fs/fuse/fuse_iomap_i.h
> >> @@ -8,17 +8,28 @@
> >>  
> >>  #if IS_ENABLED(CONFIG_FUSE_IOMAP)
> >>  #if IS_ENABLED(CONFIG_FUSE_IOMAP_DEBUG)
> >> -# define ASSERT(condition) do {						\
> >> +
> >> +#if IS_ENABLED(CONFIG_FUSE_IOMAP_DEBUG_BY_DEFAULT)
> >> +DECLARE_STATIC_KEY_TRUE(fuse_iomap_debug);
> >> +#else
> >> +DECLARE_STATIC_KEY_FALSE(fuse_iomap_debug);
> >> +#endif /* FUSE_IOMAP_DEBUG_BY_DEFAULT */
> >> +
> >> +# define ASSERT(condition) \
> >> +while (static_branch_unlikely(&fuse_iomap_debug)) {			\
> >>  	int __cond = !!(condition);					\
> >>  	if (unlikely(!__cond))						\
> >>  		trace_fuse_iomap_assert(__func__, __LINE__, #condition); \
> >>  	WARN(!__cond, "Assertion failed: %s, func: %s, line: %d", #condition, __func__, __LINE__); \
> >> -} while (0)
> >> +	break;								\
> >> +}
> >>  # define BAD_DATA(condition) ({						\
> >>  	int __cond = !!(condition);					\
> >>  	if (unlikely(__cond))						\
> >>  		trace_fuse_iomap_bad_data(__func__, __LINE__, #condition); \
> >> -	WARN(__cond, "Bad mapping: %s, func: %s, line: %d", #condition, __func__, __LINE__); \
> >> +	if (static_branch_unlikely(&fuse_iomap_debug))			\
> >> +		WARN(__cond, "Bad mapping: %s, func: %s, line: %d", #condition, __func__, __LINE__); \
> >> +	unlikely(__cond);								\
> >>  })
> >>  #else
> >>  # define ASSERT(condition)
> >> diff --git a/fs/fuse/Kconfig b/fs/fuse/Kconfig
> >> index 934d48076a010c..1b8990f1c2a8f9 100644
> >> --- a/fs/fuse/Kconfig
> >> +++ b/fs/fuse/Kconfig
> >> @@ -101,6 +101,21 @@ config FUSE_IOMAP_DEBUG
> >>  	  Enable debugging assertions for the fuse iomap code paths and logging
> >>  	  of bad iomap file mapping data being sent to the kernel.
> >>  
> >> +	  Say N here if you don't want any debugging code code compiled in at
> >> +	  all.
> >> +
> >> +config FUSE_IOMAP_DEBUG_BY_DEFAULT
> >> +	bool "Debug FUSE file IO over iomap at boot time"
> >> +	default n
> >> +	depends on FUSE_IOMAP_DEBUG
> >> +	help
> >> +	  At boot time, enable debugging assertions for the fuse iomap code
> >> +	  paths and warnings about bad iomap file mapping data.  This enables
> >> +	  fuse server authors to control debugging at runtime even on a
> >> +	  distribution kernel while avoiding most of the overhead on production
> >> +	  systems.  The setting can be changed at runtime via the debug_iomap
> >> +	  module parameter.
> >> +
> >>  config FUSE_IO_URING
> >>  	bool "FUSE communication over io-uring"
> >>  	default y
> >> diff --git a/fs/fuse/fuse_iomap.c b/fs/fuse/fuse_iomap.c
> >> index c22c7961cc0bdc..f7a7eba8317c18 100644
> >> --- a/fs/fuse/fuse_iomap.c
> >> +++ b/fs/fuse/fuse_iomap.c
> >> @@ -18,6 +18,49 @@ static bool __read_mostly enable_iomap =
> >>  module_param(enable_iomap, bool, 0644);
> >>  MODULE_PARM_DESC(enable_iomap, "Enable file I/O through iomap");
> >>  
> >> +#if IS_ENABLED(CONFIG_FUSE_IOMAP_DEBUG)
> >> +#if IS_ENABLED(CONFIG_FUSE_IOMAP_DEBUG_BY_DEFAULT)
> >> +DEFINE_STATIC_KEY_TRUE(fuse_iomap_debug);
> >> +#else
> >> +DEFINE_STATIC_KEY_FALSE(fuse_iomap_debug);
> >> +#endif /* FUSE_IOMAP_DEBUG_BY_DEFAULT */
> >> +
> >> +static int iomap_debug_set(const char *val, const struct kernel_param *kp)
> >> +{
> >> +	bool now;
> >> +	int ret;
> >> +
> >> +	if (!val)
> >> +		return -EINVAL;
> >> +
> >> +	ret = kstrtobool(val, &now);
> >> +	if (ret)
> >> +		return ret;
> >> +
> >> +	if (now)
> >> +		static_branch_enable(&fuse_iomap_debug);
> >> +	else
> >> +		static_branch_disable(&fuse_iomap_debug);
> >> +
> >> +	return 0;
> >> +}
> >> +
> >> +static int iomap_debug_get(char *buffer, const struct kernel_param *kp)
> >> +{
> >> +	return sprintf(buffer, "%c\n",
> >> +		       static_branch_unlikely(&fuse_iomap_debug) ? 'Y' : 'N');
> >> +}
> >> +
> >> +static const struct kernel_param_ops iomap_debug_ops = {
> >> +	.set = iomap_debug_set,
> >> +	.get = iomap_debug_get,
> > 
> > Codex points out that if we set KERNEL_PARAM_OPS_FL_NOARG in
> > kernel_param_ops::flags and teach iomap_debug_set to handle !val as now
> > == true, then we can support enabling debugging with
> > "modprobe fuse debug_iomap".  That seems reasonable to me, so I'll make
> > this minor change.
> > 
> > --D
> > 
> >> +};
> >> +
> >> +module_param_cb(debug_iomap, &iomap_debug_ops, NULL, 0644);
> >> +__MODULE_PARM_TYPE(debug_iomap, "bool");
> >> +MODULE_PARM_DESC(debug_iomap, "Enable debugging of fuse iomap");
> >> +#endif /* IS_ENABLED(CONFIG_FUSE_IOMAP_DEBUG) */
> >> +
> >>  bool fuse_iomap_enabled(void)
> >>  {
> >>  	/* Don't let anyone touch iomap until the end of the patchset. */
> >>
> >>
> 
> 
> Hi Darrick,
> 
> would it help to just use debugfs? I will try to find time to send that
> out tomorrow.
> 
> https://github.com/bsbernd/linux/commit/5f25afb42ae7bb7ead646cfa3a3683817975ccb9
> 
> (on this branch https://github.com/bsbernd/linux/commits/fuse-debugfs/)

I'm skeptical, because debugfs is disabled in lockdown mode.  Presumably
some $enterprise customers might want to turn on the extra logging and
assertions even in production?  Some product groups I work with require
secureboot even in their dev environments.

--D

> 
> Thanks,
> Bernd
> 
> Thanks,
> Bernd
> 
>