[PATCH] KVM: selftests: Make guest_snprintf() a macro and use it as appropriate

[PATCH] KVM: selftests: Make guest_snprintf() a macro and use it as appropriate

[Sean Christopherson]

Turn guest_snprintf() into a macro so that it can be used by variadic
functions, and use it in the ucall APIs instead of open coding the VA goo.

Note, using a macro drops the __printf() validation, but that's a non-issue
because in practice the caller will have its own validation, as is the case
with both ucall_assert() and ucall_fmt().

Reported-by: Gokul K <gokul02k@gmail.com>
Closes: https://lore.kernel.org/all/20260918113631.634233-4-gokul02k@gmail.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 tools/testing/selftests/kvm/include/test_util.h | 12 +++++++++++-
 tools/testing/selftests/kvm/lib/guest_sprintf.c | 12 ------------
 tools/testing/selftests/kvm/lib/ucall_common.c  | 10 ++--------
 3 files changed, 13 insertions(+), 21 deletions(-)

diff --git a/tools/testing/selftests/kvm/include/test_util.h b/tools/testing/selftests/kvm/include/test_util.h
index 3a432499b338..9ea5b8906dd4 100644
--- a/tools/testing/selftests/kvm/include/test_util.h
+++ b/tools/testing/selftests/kvm/include/test_util.h
@@ -238,7 +238,17 @@ static inline u32 atoi_non_negative(const char *name, const char *num_str)
 }
 
 int guest_vsnprintf(char *buf, int n, const char *fmt, va_list args);
-__printf(3, 4) int guest_snprintf(char *buf, int n, const char *fmt, ...);
+
+#define guest_snprintf(__buf, __n, __fmt)		\
+({							\
+	va_list va;					\
+	int len;					\
+							\
+	va_start(va, __fmt);				\
+	len = guest_vsnprintf(__buf, __n, __fmt, va);	\
+	va_end(va);					\
+	len;						\
+})
 
 char *strdup_printf(const char *fmt, ...) __attribute__((format(printf, 1, 2), nonnull(1)));
 
diff --git a/tools/testing/selftests/kvm/lib/guest_sprintf.c b/tools/testing/selftests/kvm/lib/guest_sprintf.c
index 7a33965349a7..6caebb456f3d 100644
--- a/tools/testing/selftests/kvm/lib/guest_sprintf.c
+++ b/tools/testing/selftests/kvm/lib/guest_sprintf.c
@@ -300,15 +300,3 @@ int guest_vsnprintf(char *buf, int n, const char *fmt, va_list args)
 	*str = '\0';
 	return str - buf;
 }
-
-int guest_snprintf(char *buf, int n, const char *fmt, ...)
-{
-	va_list va;
-	int len;
-
-	va_start(va, fmt);
-	len = guest_vsnprintf(buf, n, fmt, va);
-	va_end(va);
-
-	return len;
-}
diff --git a/tools/testing/selftests/kvm/lib/ucall_common.c b/tools/testing/selftests/kvm/lib/ucall_common.c
index 029ce21f9f2f..e9df71197367 100644
--- a/tools/testing/selftests/kvm/lib/ucall_common.c
+++ b/tools/testing/selftests/kvm/lib/ucall_common.c
@@ -83,7 +83,6 @@ void ucall_assert(u64 cmd, const char *exp, const char *file,
 		  unsigned int line, const char *fmt, ...)
 {
 	struct ucall *uc;
-	va_list va;
 
 	uc = ucall_alloc();
 	uc->cmd = cmd;
@@ -92,9 +91,7 @@ void ucall_assert(u64 cmd, const char *exp, const char *file,
 	WRITE_ONCE(uc->args[GUEST_FILE], (u64)(file));
 	WRITE_ONCE(uc->args[GUEST_LINE], line);
 
-	va_start(va, fmt);
-	guest_vsnprintf(uc->buffer, UCALL_BUFFER_LEN, fmt, va);
-	va_end(va);
+	guest_snprintf(uc->buffer, UCALL_BUFFER_LEN, fmt);
 
 	ucall_arch_do_ucall((gva_t)uc->hva);
 
@@ -104,14 +101,11 @@ void ucall_assert(u64 cmd, const char *exp, const char *file,
 void ucall_fmt(u64 cmd, const char *fmt, ...)
 {
 	struct ucall *uc;
-	va_list va;
 
 	uc = ucall_alloc();
 	uc->cmd = cmd;
 
-	va_start(va, fmt);
-	guest_vsnprintf(uc->buffer, UCALL_BUFFER_LEN, fmt, va);
-	va_end(va);
+	guest_snprintf(uc->buffer, UCALL_BUFFER_LEN, fmt);
 
 	ucall_arch_do_ucall((gva_t)uc->hva);
 

base-commit: a0bc8e1d7a82bb143b8f8f44ae3a01938f37c2ea
-- 
2.56.0.rc1.315.gc6ed9934b7-goog

Re: [PATCH] KVM: selftests: Make guest_snprintf() a macro and use it as appropriate

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [Medium] selftests/kvm: guest_snprintf() macro variables shadow caller arguments
- [Medium] selftests/kvm: guest_snprintf() implicitly captures caller variadic args
--

commit fab1360854d5b8141536eb1f70a7ed77983d6dcd
Author: Sean Christopherson <seanjc@google.com>

KVM: selftests: Make guest_snprintf() a macro and use it as appropriate

This commit turns guest_snprintf() into a macro to allow its use by variadic
functions. It updates the ucall APIs to use this macro instead of open coding
the variable argument handling.

Closes: https://lore.kernel.org/all/20260918113631.634233-4-gokul02k@gmail.com

> diff --git a/tools/testing/selftests/kvm/include/test_util.h b/tools/testing/selftests/kvm/include/test_util.h
> index a6a3e16578959..817d4d5ca7cc7 100644
> --- a/tools/testing/selftests/kvm/include/test_util.h
> +++ b/tools/testing/selftests/kvm/include/test_util.h
> @@ -238,7 +238,17 @@ static inline u32 atoi_non_negative(const char *name, const char *num_str)
>  }
>  
>  int guest_vsnprintf(char *buf, int n, const char *fmt, va_list args);
> -__printf(3, 4) int guest_snprintf(char *buf, int n, const char *fmt, ...);
> +
> +#define guest_snprintf(__buf, __n, __fmt)		\
> +({							\
> +	va_list va;					\
> +	int len;					\

[Severity: Medium]
Does this macro shadow caller arguments? If a caller passes an argument
named len to guest_snprintf(), it will be shadowed by the uninitialized
local len due to C scoping rules for statement expressions. The inner len
is then passed to guest_vsnprintf(), which could result in a random or
uninitialized buffer size.

> +							\
> +	va_start(va, __fmt);				\

[Severity: Medium]
Does this implicitly capture the caller's variadic arguments? 

The macro internally invokes va_start(va, __fmt), which couples it to the
caller's stack frame. If this macro is used in a non-variadic function,
or if __fmt is not the rightmost named argument of the calling function,
could va_start cause undefined behavior? 

Additionally, this restricts the macro to exactly 3 arguments, preventing
standard snprintf usage like guest_snprintf(buf, size, "%d", 5).

> +	len = guest_vsnprintf(__buf, __n, __fmt, va);	\
> +	va_end(va);					\
> +	len;						\
> +})
>  
>  char *strdup_printf(const char *fmt, ...) __attribute__((format(printf, 1, 2), nonnull(1)));

[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260930001341.3171558-1-seanjc@google.com?part=1

Re: [PATCH] KVM: selftests: Make guest_snprintf() a macro and use it as appropriate

[Sean Christopherson]

On Wed, Sep 30, 2026, sashiko-bot@kernel.org wrote:
> Closes: https://lore.kernel.org/all/20260918113631.634233-4-gokul02k@gmail.com
> 
> > diff --git a/tools/testing/selftests/kvm/include/test_util.h b/tools/testing/selftests/kvm/include/test_util.h
> > index a6a3e16578959..817d4d5ca7cc7 100644
> > --- a/tools/testing/selftests/kvm/include/test_util.h
> > +++ b/tools/testing/selftests/kvm/include/test_util.h
> > @@ -238,7 +238,17 @@ static inline u32 atoi_non_negative(const char *name, const char *num_str)
> >  }
> >  
> >  int guest_vsnprintf(char *buf, int n, const char *fmt, va_list args);
> > -__printf(3, 4) int guest_snprintf(char *buf, int n, const char *fmt, ...);
> > +
> > +#define guest_snprintf(__buf, __n, __fmt)		\
> > +({							\
> > +	va_list va;					\
> > +	int len;					\
> 
> [Severity: Medium]
> Does this macro shadow caller arguments? If a caller passes an argument
> named len to guest_snprintf(), it will be shadowed by the uninitialized
> local len due to C scoping rules for statement expressions. The inner len
> is then passed to guest_vsnprintf(), which could result in a random or
> uninitialized buffer size.
> 
> > +							\
> > +	va_start(va, __fmt);				\
> 
> [Severity: Medium]
> Does this implicitly capture the caller's variadic arguments? 
> 
> The macro internally invokes va_start(va, __fmt), which couples it to the
> caller's stack frame. If this macro is used in a non-variadic function,
> or if __fmt is not the rightmost named argument of the calling function,
> could va_start cause undefined behavior? 

Ugh, and now I remember why we ended up with duplicate code.  Lame.  I'll drop
this and grab Gokul's original patch.  As much as I want to keep guest_snprintf()
around "just in case", in practice I highly doubt we'll ever use it.