Re: [PATCHv4 bpf-next 2/5] libbpf: Add support to detect nop,nop5 instructions combo for usdt probe

[Jiri Olsa]

On Sat, Feb 21, 2026 at 12:00:35PM +0000, David Laight wrote:
> On Fri, 20 Feb 2026 11:24:45 +0000 (UTC)
> bot+bpf-ci@xxxxxxxxxx wrote:
> 
> > > diff --git a/tools/lib/bpf/usdt.c b/tools/lib/bpf/usdt.c
> > > index d1524f6f54ae..c81e0a58caf5 100644
> > > --- a/tools/lib/bpf/usdt.c
> > > +++ b/tools/lib/bpf/usdt.c  
> > 
> > [ ... ]
> > 
> > > +#if defined(__x86_64__)
> > > +static bool has_nop_combo(int fd, long off)
> > > +{
> > > +	static unsigned char nop_combo[6] = {  
> >            ^^^^^^^^^^^^^^^^^^^^^^
> > 
> > Should nop_combo be declared const? It is a read-only lookup table
> > used only in the memcmp() below.
> 
> Actually you either want 'static const unsigned char' or just 'unsigned char'.
> In either case gcc optimises away the array, the call to memcmp(), and
> just compares with 32bit and 64bit constants.

ok, makes sense, will change

> 
> > 
> > > +		0x90, 0x0f, 0x1f, 0x44, 0x00, 0x00 /* nop,nop5 */
> > > +	};
> > > +	unsigned char buf[6] = {};
> 
> No need to initialise buf[];

right, thanks,
jirka


> 
> 	David
> 
> > > +
> > > +	if (pread(fd, buf, 6, off) != 6)
> > > +		return false;
> > > +	return memcmp(buf, nop_combo, 6) == 0;
> > > +}  
> > 
> > [ ... ]
> > 
> > 
> > ---
> > AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> > See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
> > 
> > CI run summary: https://github.com/kernel-patches/bpf/actions/runs/22221513473
> > 
> > AI-authorship-score: low
> > AI-authorship-explanation: Domain-specific optimization from established BPF/tracing maintainer with iterative lore review history and deep x86_64 uprobe internals knowledge.
> > issues-found: 1
> > issue-severity-score: low
> > issue-severity-explanation: Missing const qualifier on a static read-only byte array is a minor C best practice deviation with no runtime impact.
>