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

[Gary Guo]

On Fri, 11 Oct 2024 23:01:12 +0100
Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:

> On Fri, Oct 11, 2024 at 10:38:53PM +0800, Zheng Yejian wrote:
> > 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.  
> 
> Technically, at least two.  If we ever have a symbol larger than
> 16kB, we'll use three bytes.

Let's not worry about things that would not happen.

scripts/kallsyms.c have a check to ensure that symbol names don't get
longer than 0x3FFF.

Best,
Gary

> 
> > +++ 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++;  
> 
> So this "if" should be a "while" for maximum future proofing against the
> day that we have a 16kB function ...
>