Re: [PATCH v2] libpf: Fix for the potential undefined behavior due to shifting

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The patch attempts to fix undefined behavior in `BTF_INFO_ENC` but places the cast incorrectly, leaving the undefined behavior unresolved. According to the C standard (C11 6.5.3.3), the logical negation operator `!` always yields a signed `int` (0 or 1), regardless of the operand's type. Thus, `!!((__u32)kind_flag)` evaluates to a signed `int`. Left-shifting this signed `int` by 31 places (`!!(...) << 31`) continues to trigger undefined behavior by shifting a bit into the sign bit of a 32-bit signed integer. To correctly prevent the undefined behavior, the cast must be applied after the logical negation, e.g., `((__u32)!!(kind_flag) << 31)`.
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commit ce021dde7807eb8995ad265d12e0e7372294cfb7
Author: Tw <tw19881113@gmail.com>

libpf: Fix for the potential undefined behavior due to shifting

This patch aims to fix an undefined behavior issue reported by UBSAN in
libbpf ("left shift of 1 by 31 places cannot be represented in type 'int'")
by casting the operand to an unsigned type before shifting.

[ ... ]

> diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
> index 4c46d34fc055d..2df1df3f3f22c 100644
> --- a/tools/lib/bpf/libbpf_internal.h
> +++ b/tools/lib/bpf/libbpf_internal.h
> @@ -77,7 +77,7 @@
>  #define JUMPTABLES_SEC ".jumptables"
>  
>  #define BTF_INFO_ENC(kind, kind_flag, vlen) \
> -	((!!(kind_flag) << 31) | ((kind) << 24) | ((vlen) & BTF_MAX_VLEN))
> +	((!!((__u32)kind_flag) << 31) | ((kind) << 24) | ((vlen) & BTF_MAX_VLEN))

[Severity: Medium]
Does this cast actually resolve the undefined behavior, or is the regression 
still present?

According to C11 6.5.3.3, the logical negation operator (!) always yields a 
signed int (0 or 1), regardless of the operand's type. This means 
!!((__u32)kind_flag) evaluates to a signed int.

Left-shifting this signed int by 31 places continues to shift a bit into 
the sign bit of a 32-bit signed integer.

Could the cast be applied after the logical negation instead, such as 
((__u32)!!(kind_flag) << 31)?

>  #define BTF_TYPE_ENC(name, info, size_or_type) (name), (info), (size_or_type)
>  #define BTF_INT_ENC(encoding, bits_offset, nr_bits) \
>  	((encoding) << 24 | (bits_offset) << 16 | (nr_bits))

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Sashiko AI review · https://sashiko.dev/#/patchset/20260827001007.1420507-1-tw19881113@gmail.com?part=1