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

[sashiko-bot@kernel.org]

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
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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)));

[ ... ]

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