[PATCH] libbpf: fix signed multiplication overflow in hash_combine

[PATCH] libbpf: fix signed multiplication overflow in hash_combine

[psykose]

when using -fsanitize=undefined (which flags signed overflow which is
UB), a crash can be reproduced when building the linux kernel with BTF
info.

cast to unsigned first to make the overflow not invoke UB semantics- the
result is the same.

Signed-off-by: psykose <alice@ayaya.dev>
---
 src/btf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/btf.c b/src/btf.c
index 2d0840e..60cd412 100644
--- a/src/btf.c
+++ b/src/btf.c
@@ -3317,7 +3317,7 @@ struct btf_dedup {
 
 static long hash_combine(long h, long value)
 {
-	return h * 31 + value;
+	return (long)((unsigned long)h * 31 + (unsigned long)value);
 }
 
 #define for_each_dedup_cand(d, node, hash) \

base-commit: 42065ea6627ff6e1ab4c65e51042a70fbf30ff7c
-- 
2.45.2

Re: [PATCH] libbpf: fix signed multiplication overflow in hash_combine

[Yonghong Song]

On 6/16/24 3:11 PM, psykose wrote:
> when using -fsanitize=undefined (which flags signed overflow which is
> UB), a crash can be reproduced when building the linux kernel with BTF
> info.
>
> cast to unsigned first to make the overflow not invoke UB semantics- the
> result is the same.
>
> Signed-off-by: psykose <alice@ayaya.dev>

This seems against upstream libbpf repo. Could you do a proper patch
against bpf-next tree. Please have details how to reproduce the failure.

Please use proper name in your Signed-off-by.

> ---
>   src/btf.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/src/btf.c b/src/btf.c
> index 2d0840e..60cd412 100644
> --- a/src/btf.c
> +++ b/src/btf.c
> @@ -3317,7 +3317,7 @@ struct btf_dedup {
>   
>   static long hash_combine(long h, long value)
>   {
> -	return h * 31 + value;
> +	return (long)((unsigned long)h * 31 + (unsigned long)value);
>   }
>   
>   #define for_each_dedup_cand(d, node, hash) \
>
> base-commit: 42065ea6627ff6e1ab4c65e51042a70fbf30ff7c