Re: [PATCH v2] kallsyms: Fix wrong "big" kernel symbol type read from procfs

[Gary Guo]

On Fri, 11 Oct 2024 22:38:53 +0800
Zheng Yejian <zhengyejian@xxxxxxxxxxxxxxx> wrote:

> Currently when the length of a symbol is longer than 0x7f characters,
> its type shown in /proc/kallsyms can be incorrect.
> 
> I found this issue when reading the code, but it can be reproduced by
> following steps:
> 
>   1. Define a function which symbol length is 130 characters:
> 
>     #define X13(x) x##x##x##x##x##x##x##x##x##x##x##x##x
>     static noinline void X13(x123456789)(void)
>     {
>         printk("hello world\n");
>     }
> 
>   2. The type in vmlinux is 't':
> 
>     $ nm vmlinux | grep x123456
>     ffffffff816290f0 t x123456789x123456789x123456789x12[...]
> 
>   3. Then boot the kernel, the type shown in /proc/kallsyms becomes 'g'
>      instead of the expected 't':
> 
>     # cat /proc/kallsyms | grep x123456
>     ffffffff816290f0 g x123456789x123456789x123456789x12[...]
> 
> The root cause is that, after commit 73bbb94466fd ("kallsyms: support
> "big" kernel symbols"), ULEB128 was used to encode symbol name length.
> That is, for "big" kernel symbols of which name length is longer than
> 0x7f characters, the length info is encoded into 2 bytes.
> 
> kallsyms_get_symbol_type() expects to read the first char of the
> symbol name which indicates the symbol type. However, due to the
> "big" symbol case not being handled, the symbol type read from
> /proc/kallsyms may be wrong, so handle it properly.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 73bbb94466fd ("kallsyms: support "big" kernel symbols")
> Signed-off-by: Zheng Yejian <zhengyejian@xxxxxxxxxxxxxxx>

Acked-by: Gary Guo <gary@xxxxxxxxxxx>

> ---
> 
> v1 -> v2:
> - Add reproduction info into commit message to make it clearer;
> - Add cc: stable line;
> 
> v1: https://lore.kernel.org/all/20240830062935.1187613-1-zhengyejian@xxxxxxxxxxxxxxx/
> 
>  kernel/kallsyms.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> index a9a0ca605d4a..9e4bf061bb83 100644
> --- a/kernel/kallsyms.c
> +++ b/kernel/kallsyms.c
> @@ -103,8 +103,11 @@ static char kallsyms_get_symbol_type(unsigned int off)
>  {
>  	/*
>  	 * Get just the first code, look it up in the token table,
> -	 * and return the first char from this token.
> +	 * and return the first char from this token. If MSB of length
> +	 * is 1, it is a "big" symbol, so needs an additional byte.
>  	 */
> +	if (kallsyms_names[off] & 0x80)
> +		off++;
>  	return kallsyms_token_table[kallsyms_token_index[kallsyms_names[off + 1]]];
>  }
>