[PATCH 1/2] objtool: fix the check for dead_end function with multiple sibliing calls

[PATCH 1/2] objtool: fix the check for dead_end function with multiple sibliing calls

[xur]

From: Rong Xu <xur@google.com>

If a function has multiple sibling calls, the dead_end check should
only return true if all sibling call targets are also dead_end
functions.

Signed-off-by: Rong Xu <xur@google.com>
Reviewed-by: Sriraman Tallam <tmsriram@google.com>
Reviewed-by: Han Shen <shenhan@google.com>
Reviewed-by: Krzysztof Pszeniczny <kpszeniczny@google.com>
---
 tools/objtool/check.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 9004fbc067693..c2ee3c3a84a62 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -314,7 +314,13 @@ static bool __dead_end_function(struct objtool_file *file, struct symbol *func,
 				return false;
 			}
 
-			return __dead_end_function(file, insn_func(dest), recursion+1);
+			/*
+			 * A function can have multiple sibling calls. All of
+			 * them need to be dead ends for the function to be a
+			 * dead end too.
+			 */
+			if (!__dead_end_function(file, insn_func(dest), recursion+1))
+				return false;
 		}
 	}
 

base-commit: 6146a0f1dfae5d37442a9ddcba012add260bceb0
-- 
2.51.2.997.g839fc31de9-goog

[PATCH 2/2] objtool: dead_end function change for split functions

[xur]

From: Rong Xu <xur@google.com>

Function Splitting can potentially move all return instructions
into the cold (infrequently executed) section of the function.
If this happens, the original function might be incorrectly
flagged as a dead-end function.

The consequence is an incomplete ORC table, which leads to an unwind
error, and subsequently, a livepatch failure.

This patch adds the support of the dead_end_function check for
split function.

Signed-off-by: Rong Xu <xur@google.com>
Reviewed-by: Sriraman Tallam <tmsriram@google.com>
Reviewed-by: Han Shen <shenhan@google.com>
Reviewed-by: Krzysztof Pszeniczny <kpszeniczny@google.com>
---
 tools/objtool/check.c | 88 +++++++++++++++++++++++++++++++++----------
 1 file changed, 69 insertions(+), 19 deletions(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index c2ee3c3a84a62..b752cf508d09a 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -237,6 +237,73 @@ static bool is_rust_noreturn(const struct symbol *func)
 		str_ends_with(func->name, "_fail"));
 }
 
+static bool __dead_end_function(struct objtool_file *, struct symbol *, int);
+
+/*
+ * Check if the target of a sibling_call instruction is a dead_end function.
+ * Note insn must be a sibling call.
+ */
+static inline bool __dead_end_sibling_call(struct objtool_file *file,
+		struct instruction *insn, int recursion) {
+	struct instruction *dest = insn->jump_dest;
+
+	if (!dest)
+		/* sibling call to another file */
+		return false;
+
+	/* local sibling call */
+	if (recursion == 5) {
+		/*
+		 * Infinite recursion: two functions have
+		 * sibling calls to each other.  This is a very
+		 * rare case.  It means they aren't dead ends.
+		 */
+		return false;
+	}
+
+	return __dead_end_function(file, insn_func(dest), recursion+1);
+}
+
+/*
+ * Handling split functions. Mimic the workflow in __dead_end_function.
+ */
+static bool __dead_end_split_func(struct objtool_file *file,
+			struct symbol *func, int recursion)
+{
+	char section_name[256];
+	struct section *sec;
+	struct instruction *insn;
+
+	/*
+	 * Use a fixed-size buffer (max 256) to avoid malloc. If the section
+	 * length exceeds this limit, we return a conservative value. This is
+	 * a safe fallback and does not compromise functional correctness.
+	 */
+	if (snprintf(section_name, sizeof(section_name), ".text.split.%s",
+		     func->name) >= sizeof(section_name)) {
+		fprintf(stderr, "Error: Function name '%s' too long.\n", func->name);
+		return false;
+	}
+
+	sec = find_section_by_name(file->elf, section_name);
+	if (!sec)
+		return false;
+
+	sec_for_each_insn(file, sec, insn) {
+		if (insn->type == INSN_RETURN)
+			return false;
+	}
+
+	sec_for_each_insn(file, sec, insn) {
+		if (is_sibling_call(insn)) {
+			if (!__dead_end_sibling_call(file, insn, recursion))
+				return false;
+		}
+	}
+
+	return true;
+}
+
 /*
  * This checks to see if the given function is a "noreturn" function.
  *
@@ -298,33 +365,16 @@ static bool __dead_end_function(struct objtool_file *file, struct symbol *func,
 	 */
 	func_for_each_insn(file, func, insn) {
 		if (is_sibling_call(insn)) {
-			struct instruction *dest = insn->jump_dest;
-
-			if (!dest)
-				/* sibling call to another file */
-				return false;
-
-			/* local sibling call */
-			if (recursion == 5) {
-				/*
-				 * Infinite recursion: two functions have
-				 * sibling calls to each other.  This is a very
-				 * rare case.  It means they aren't dead ends.
-				 */
-				return false;
-			}
-
 			/*
 			 * A function can have multiple sibling calls. All of
 			 * them need to be dead ends for the function to be a
 			 * dead end too.
 			 */
-			if (!__dead_end_function(file, insn_func(dest), recursion+1))
+			if (!__dead_end_sibling_call(file, insn, recursion))
 				return false;
 		}
 	}
-
-	return true;
+	return __dead_end_split_func(file, func, recursion);
 }
 
 static bool dead_end_function(struct objtool_file *file, struct symbol *func)
-- 
2.51.2.997.g839fc31de9-goog

Re: [PATCH 2/2] objtool: dead_end function change for split functions

[Peter Zijlstra]

On Mon, Nov 03, 2025 at 06:51:54PM +0000, xur@google.com wrote:
> From: Rong Xu <xur@google.com>
> 
> Function Splitting can potentially move all return instructions
> into the cold (infrequently executed) section of the function.
> If this happens, the original function might be incorrectly
> flagged as a dead-end function.
> 
> The consequence is an incomplete ORC table, which leads to an unwind
> error, and subsequently, a livepatch failure.
> 
> This patch adds the support of the dead_end_function check for
> split function.
> 
> Signed-off-by: Rong Xu <xur@google.com>
> Reviewed-by: Sriraman Tallam <tmsriram@google.com>
> Reviewed-by: Han Shen <shenhan@google.com>
> Reviewed-by: Krzysztof Pszeniczny <kpszeniczny@google.com>
> ---
>  tools/objtool/check.c | 88 +++++++++++++++++++++++++++++++++----------
>  1 file changed, 69 insertions(+), 19 deletions(-)
> 
> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index c2ee3c3a84a62..b752cf508d09a 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -237,6 +237,73 @@ static bool is_rust_noreturn(const struct symbol *func)
>  		str_ends_with(func->name, "_fail"));
>  }
>  
> +static bool __dead_end_function(struct objtool_file *, struct symbol *, int);
> +
> +/*
> + * Check if the target of a sibling_call instruction is a dead_end function.
> + * Note insn must be a sibling call.
> + */
> +static inline bool __dead_end_sibling_call(struct objtool_file *file,
> +		struct instruction *insn, int recursion) {

Please: cino=(0:0
also for functions { on a new line.

> +	struct instruction *dest = insn->jump_dest;
> +
> +	if (!dest)
> +		/* sibling call to another file */
> +		return false;

I know this is just code movement, but this wants {} per coding style.

> +
> +	/* local sibling call */
> +	if (recursion == 5) {
> +		/*
> +		 * Infinite recursion: two functions have
> +		 * sibling calls to each other.  This is a very
> +		 * rare case.  It means they aren't dead ends.
> +		 */
> +		return false;
> +	}
> +
> +	return __dead_end_function(file, insn_func(dest), recursion+1);
> +}
> +
> +/*
> + * Handling split functions. Mimic the workflow in __dead_end_function.
> + */
> +static bool __dead_end_split_func(struct objtool_file *file,
> +			struct symbol *func, int recursion)

cino=(0:0

> +{
> +	char section_name[256];
> +	struct section *sec;
> +	struct instruction *insn;
> +
> +	/*
> +	 * Use a fixed-size buffer (max 256) to avoid malloc. If the section
> +	 * length exceeds this limit, we return a conservative value. This is
> +	 * a safe fallback and does not compromise functional correctness.
> +	 */
> +	if (snprintf(section_name, sizeof(section_name), ".text.split.%s",
> +		     func->name) >= sizeof(section_name)) {

That is a terribly confusing line-break to read. Might've been better to
split after the greate-or-equal sign.

> +		fprintf(stderr, "Error: Function name '%s' too long.\n", func->name);
> +		return false;
> +	}
> +
> +	sec = find_section_by_name(file->elf, section_name);
> +	if (!sec)
> +		return false;
> +
> +	sec_for_each_insn(file, sec, insn) {
> +		if (insn->type == INSN_RETURN)
> +			return false;
> +	}
> +
> +	sec_for_each_insn(file, sec, insn) {
> +		if (is_sibling_call(insn)) {
> +			if (!__dead_end_sibling_call(file, insn, recursion))
> +				return false;
> +		}
> +	}
> +
> +	return true;
> +}
> +
>  /*
>   * This checks to see if the given function is a "noreturn" function.
>   *
> @@ -298,33 +365,16 @@ static bool __dead_end_function(struct objtool_file *file, struct symbol *func,
>  	 */
>  	func_for_each_insn(file, func, insn) {
>  		if (is_sibling_call(insn)) {
> -			struct instruction *dest = insn->jump_dest;
> -
> -			if (!dest)
> -				/* sibling call to another file */
> -				return false;
> -
> -			/* local sibling call */
> -			if (recursion == 5) {
> -				/*
> -				 * Infinite recursion: two functions have
> -				 * sibling calls to each other.  This is a very
> -				 * rare case.  It means they aren't dead ends.
> -				 */
> -				return false;
> -			}
> -
>  			/*
>  			 * A function can have multiple sibling calls. All of
>  			 * them need to be dead ends for the function to be a
>  			 * dead end too.
>  			 */
> -			if (!__dead_end_function(file, insn_func(dest), recursion+1))
> +			if (!__dead_end_sibling_call(file, insn, recursion))
>  				return false;
>  		}
>  	}
> -
> -	return true;
> +	return __dead_end_split_func(file, func, recursion);
>  }

Aside from some coding style nits, this seems like it will do.

Re: [PATCH 2/2] objtool: dead_end function change for split functions

[Rong Xu]

Peter, thanks for the review! I will fix the style and send patch v2.

-Rong

On Mon, Nov 3, 2025 at 1:19 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Mon, Nov 03, 2025 at 06:51:54PM +0000, xur@google.com wrote:
> > From: Rong Xu <xur@google.com>
> >
> > Function Splitting can potentially move all return instructions
> > into the cold (infrequently executed) section of the function.
> > If this happens, the original function might be incorrectly
> > flagged as a dead-end function.
> >
> > The consequence is an incomplete ORC table, which leads to an unwind
> > error, and subsequently, a livepatch failure.
> >
> > This patch adds the support of the dead_end_function check for
> > split function.
> >
> > Signed-off-by: Rong Xu <xur@google.com>
> > Reviewed-by: Sriraman Tallam <tmsriram@google.com>
> > Reviewed-by: Han Shen <shenhan@google.com>
> > Reviewed-by: Krzysztof Pszeniczny <kpszeniczny@google.com>
> > ---
> >  tools/objtool/check.c | 88 +++++++++++++++++++++++++++++++++----------
> >  1 file changed, 69 insertions(+), 19 deletions(-)
> >
> > diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> > index c2ee3c3a84a62..b752cf508d09a 100644
> > --- a/tools/objtool/check.c
> > +++ b/tools/objtool/check.c
> > @@ -237,6 +237,73 @@ static bool is_rust_noreturn(const struct symbol *func)
> >               str_ends_with(func->name, "_fail"));
> >  }
> >
> > +static bool __dead_end_function(struct objtool_file *, struct symbol *, int);
> > +
> > +/*
> > + * Check if the target of a sibling_call instruction is a dead_end function.
> > + * Note insn must be a sibling call.
> > + */
> > +static inline bool __dead_end_sibling_call(struct objtool_file *file,
> > +             struct instruction *insn, int recursion) {
>
> Please: cino=(0:0
> also for functions { on a new line.
>
> > +     struct instruction *dest = insn->jump_dest;
> > +
> > +     if (!dest)
> > +             /* sibling call to another file */
> > +             return false;
>
> I know this is just code movement, but this wants {} per coding style.
>
> > +
> > +     /* local sibling call */
> > +     if (recursion == 5) {
> > +             /*
> > +              * Infinite recursion: two functions have
> > +              * sibling calls to each other.  This is a very
> > +              * rare case.  It means they aren't dead ends.
> > +              */
> > +             return false;
> > +     }
> > +
> > +     return __dead_end_function(file, insn_func(dest), recursion+1);
> > +}
> > +
> > +/*
> > + * Handling split functions. Mimic the workflow in __dead_end_function.
> > + */
> > +static bool __dead_end_split_func(struct objtool_file *file,
> > +                     struct symbol *func, int recursion)
>
> cino=(0:0
>
> > +{
> > +     char section_name[256];
> > +     struct section *sec;
> > +     struct instruction *insn;
> > +
> > +     /*
> > +      * Use a fixed-size buffer (max 256) to avoid malloc. If the section
> > +      * length exceeds this limit, we return a conservative value. This is
> > +      * a safe fallback and does not compromise functional correctness.
> > +      */
> > +     if (snprintf(section_name, sizeof(section_name), ".text.split.%s",
> > +                  func->name) >= sizeof(section_name)) {
>
> That is a terribly confusing line-break to read. Might've been better to
> split after the greate-or-equal sign.
>
> > +             fprintf(stderr, "Error: Function name '%s' too long.\n", func->name);
> > +             return false;
> > +     }
> > +
> > +     sec = find_section_by_name(file->elf, section_name);
> > +     if (!sec)
> > +             return false;
> > +
> > +     sec_for_each_insn(file, sec, insn) {
> > +             if (insn->type == INSN_RETURN)
> > +                     return false;
> > +     }
> > +
> > +     sec_for_each_insn(file, sec, insn) {
> > +             if (is_sibling_call(insn)) {
> > +                     if (!__dead_end_sibling_call(file, insn, recursion))
> > +                             return false;
> > +             }
> > +     }
> > +
> > +     return true;
> > +}
> > +
> >  /*
> >   * This checks to see if the given function is a "noreturn" function.
> >   *
> > @@ -298,33 +365,16 @@ static bool __dead_end_function(struct objtool_file *file, struct symbol *func,
> >        */
> >       func_for_each_insn(file, func, insn) {
> >               if (is_sibling_call(insn)) {
> > -                     struct instruction *dest = insn->jump_dest;
> > -
> > -                     if (!dest)
> > -                             /* sibling call to another file */
> > -                             return false;
> > -
> > -                     /* local sibling call */
> > -                     if (recursion == 5) {
> > -                             /*
> > -                              * Infinite recursion: two functions have
> > -                              * sibling calls to each other.  This is a very
> > -                              * rare case.  It means they aren't dead ends.
> > -                              */
> > -                             return false;
> > -                     }
> > -
> >                       /*
> >                        * A function can have multiple sibling calls. All of
> >                        * them need to be dead ends for the function to be a
> >                        * dead end too.
> >                        */
> > -                     if (!__dead_end_function(file, insn_func(dest), recursion+1))
> > +                     if (!__dead_end_sibling_call(file, insn, recursion))
> >                               return false;
> >               }
> >       }
> > -
> > -     return true;
> > +     return __dead_end_split_func(file, func, recursion);
> >  }
>
> Aside from some coding style nits, this seems like it will do.
>

Re: [PATCH 1/2] objtool: fix the check for dead_end function with multiple sibliing calls

[Peter Zijlstra]

On Mon, Nov 03, 2025 at 06:51:53PM +0000, xur@google.com wrote:
> From: Rong Xu <xur@google.com>
> 
> If a function has multiple sibling calls, the dead_end check should
> only return true if all sibling call targets are also dead_end
> functions.

Cute, however did you find this?

> Signed-off-by: Rong Xu <xur@google.com>
> Reviewed-by: Sriraman Tallam <tmsriram@google.com>
> Reviewed-by: Han Shen <shenhan@google.com>
> Reviewed-by: Krzysztof Pszeniczny <kpszeniczny@google.com>
> ---
>  tools/objtool/check.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index 9004fbc067693..c2ee3c3a84a62 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -314,7 +314,13 @@ static bool __dead_end_function(struct objtool_file *file, struct symbol *func,
>  				return false;
>  			}
>  
> -			return __dead_end_function(file, insn_func(dest), recursion+1);
> +			/*
> +			 * A function can have multiple sibling calls. All of
> +			 * them need to be dead ends for the function to be a
> +			 * dead end too.
> +			 */
> +			if (!__dead_end_function(file, insn_func(dest), recursion+1))
> +				return false;
>  		}
>  	}
>  
> 
> base-commit: 6146a0f1dfae5d37442a9ddcba012add260bceb0
> -- 
> 2.51.2.997.g839fc31de9-goog
> 

Re: [PATCH 1/2] objtool: fix the check for dead_end function with multiple sibliing calls

[Rong Xu]

While investigating a fix for the split function, I noticed this issue
in the code.
I'm not sure if we have multiple sibling calls in the current kernel
source. But I think
we should fix it.

The split function issue did affect us in the production.

Thanks,

-Rong

On Mon, Nov 3, 2025 at 1:01 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Mon, Nov 03, 2025 at 06:51:53PM +0000, xur@google.com wrote:
> > From: Rong Xu <xur@google.com>
> >
> > If a function has multiple sibling calls, the dead_end check should
> > only return true if all sibling call targets are also dead_end
> > functions.
>
> Cute, however did you find this?
>
> > Signed-off-by: Rong Xu <xur@google.com>
> > Reviewed-by: Sriraman Tallam <tmsriram@google.com>
> > Reviewed-by: Han Shen <shenhan@google.com>
> > Reviewed-by: Krzysztof Pszeniczny <kpszeniczny@google.com>
> > ---
> >  tools/objtool/check.c | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> > index 9004fbc067693..c2ee3c3a84a62 100644
> > --- a/tools/objtool/check.c
> > +++ b/tools/objtool/check.c
> > @@ -314,7 +314,13 @@ static bool __dead_end_function(struct objtool_file *file, struct symbol *func,
> >                               return false;
> >                       }
> >
> > -                     return __dead_end_function(file, insn_func(dest), recursion+1);
> > +                     /*
> > +                      * A function can have multiple sibling calls. All of
> > +                      * them need to be dead ends for the function to be a
> > +                      * dead end too.
> > +                      */
> > +                     if (!__dead_end_function(file, insn_func(dest), recursion+1))
> > +                             return false;
> >               }
> >       }
> >
> >
> > base-commit: 6146a0f1dfae5d37442a9ddcba012add260bceb0
> > --
> > 2.51.2.997.g839fc31de9-goog
> >