Re: [PATCH v3 0/8] perf: Support multiple system call tables in the build

[Namhyung Kim <namhyung@kernel.org>]

On Mon, Feb 24, 2025 at 08:22:50PM -0800, Ian Rogers wrote:
> On Mon, Feb 24, 2025 at 7:20 PM Namhyung Kim <namhyung@kernel.org> wrote:
> >
> > On Wed, Feb 19, 2025 at 10:56:49AM -0800, Ian Rogers wrote:
> > > This work builds on the clean up of system call tables and removal of
> > > libaudit by Charlie Jenkins <charlie@rivosinc.com>.
> > >
> > > The system call table in perf trace is used to map system call numbers
> > > to names and vice versa. Prior to these changes, a single table
> > > matching the perf binary's build was present. The table would be
> > > incorrect if tracing say a 32-bit binary from a 64-bit version of
> > > perf, the names and numbers wouldn't match.
> > >
> > > Change the build so that a single system call file is built and the
> > > potentially multiple tables are identifiable from the ELF machine type
> > > of the process being examined. To determine the ELF machine type, the
> > > executable's header is read from /proc/pid/exe with fallbacks to using
> > > the perf's binary type when unknown.
> >
> > Hmm.. then this is limited to live mode and potentially detect wrong
> > machine type if it reads an old data, right?
> >
> > Also IIUC fallback to the perf binary means it cannot use cross-machine
> > table.  For example, it cannot process data from ARM64 on x86, no?  It
> > seems it should use perf_env.arch.
> 
> The perf env arch is kind of horrid. On x86 it has the value x86 and
> then there is an extra 64bit flag, who knows how x32 should be encoded
> - but we barely support x32 as-is. I'd rather we added a new feature
> for the e_machine/e_flags of the executable and worked with those, but
> it is kind of weird with doing system wide mode. I didn't want to drag
> that into this patch series anyway as there is already enough here.

Right, I don't know how to handle x32 properly.  Maybe we can just
ignore it for now.

But anyway looking at /proc/PID for recorded data doesn't seem correct.
Can you please add a flag to do that only from trace__run() and just use
EM_HOST for trace__replay()?

Later, we may need to add a misc flag or so to PERF_RECORD_FORK (and
PERF_RECORD_COMM with MISC_COMM_EXEC) to indicate non-standard ABI for a
new thread.  But it's not clear how to make it arch-independent.

> 
> > One more concern is BPF.  The BPF should know about the ABI of the
> > current process so that it can augment the syscall arguments correctly.
> > Currently it only checks the syscall number but it can be different on
> > 32-bit and 64-bit.
> 
> That's right. This change is trying to clean up
> tools/perf/util/syscalltbl.c and the perf trace usage. I didn't go as
> far as making BPF programs pair system call number with e_machine and
> e_flags, there is enough here and the behavior after these patches
> matches the behavior before - that is to assume the system call ABI
> matches that of the perf binary.

Right, the next step would be adding a BPF kfunc to identify the current
ABI.

Thanks,
Namhyung