Re: [PATCH v2 1/1] arm64: Do not forget syscall when starting a new thread.

[Francis Laniel <flaniel@linux.microsoft.com>]

Hi.

Le mardi 28 juin 2022, 21:26:32 CEST Francis Laniel a écrit :
> Hi.
> 
> Le mardi 28 juin 2022, 15:58:35 CEST Will Deacon a écrit :
> > On Wed, Jun 08, 2022 at 05:24:46PM +0100, Francis Laniel wrote:
> > > This patch enables exeve*() to be traced with syscalls:sys_exit_execve
> > > tracepoint.
> > > Previous to it, by calling forget_syscall(), this tracepoint would not
> > > print its information as syscall is -1.
> > > So, this patch removes call to forget_syscall() and set regs->syscallno
> > > to its previous value.
> > > 
> > > Signed-off-by: Francis Laniel <flaniel@linux.microsoft.com>
> > > ---
> > > 
> > >  arch/arm64/include/asm/processor.h | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/arm64/include/asm/processor.h
> > > b/arch/arm64/include/asm/processor.h index 9e58749db21d..86eb0bfe3b38
> > > 100644
> > > --- a/arch/arm64/include/asm/processor.h
> > > +++ b/arch/arm64/include/asm/processor.h
> > > @@ -272,8 +272,9 @@ void tls_preserve_current_state(void);
> > > 
> > >  static inline void start_thread_common(struct pt_regs *regs, unsigned
> > >  long pc) {
> > > 
> > > +	s32 previous_syscall = regs->syscallno;
> > > 
> > >  	memset(regs, 0, sizeof(*regs));
> > > 
> > > -	forget_syscall(regs);
> > > +	regs->syscallno = previous_syscall;
> > 
> > I'm still unsure about this. Even if we preserve the syscall number here,
> > won't all the arguments be reported as 0?
> 
> I am not really sure what you meant about arguments, can you please precise
> between command line arguments (ls -al) and syscall arguments (argp, envp,
> etc.)?
> Indeed, if my understanding is correct syscall arguments are showed by
> sys_enter_* while sys_exit_* only reports the syscall return code.
> 
> Regarding the return code I think the value is correct as it is used in
> syscall_trace_exit() but set in invoke_syscall() after the syscall finishes
> [1, 2].
> The comparison of arm64 and amd64 output also shows no difference:
> # amd64
> ls 435739 [002] 24689.292479:  syscalls:sys_exit_execve: 0x0
>             7fc43732e100 _start+0x0 (/usr/lib/x86_64-linux-gnu/ld-2.31.so)
> # arm64
> ls   266 [000]    34.708444:  syscalls:sys_exit_execve: 0x0
>                     1140 [unknown] (/usr/lib/aarch64-linux-gnu/ld-2.31.so)
> 
> > I also looked quickly at the 32-bit arch/arm/ code and it looks like the
> > same behaviour exists there (module CONFIG_BINFMT_ELF_FDPIC).
> 

I tested arm32 and it is not affected (even though I did not have 
CONFIG_BINFMT_ELF_FDPIC set).
Here is ftrace output for arm64 without this patch:
            bash-316     [000] .....    72.167342: sys_execve(filename: 
aaaaf9bbcd30, argv: aaaaf9bb54f0, envp: aaaaf9a7d9b0)
Here is the output for arm64 with this patch:
             cat-313     [000] .....   417.926073: sys_execve(filename: 
aaaaee7ce9f0, argv: aaaaee7833a0, envp: aaaaee6a69b0)
             cat-313     [000] .....   417.939619: sys_execve -> 0x0
And here is output for arm32:
             cat-254     [000] .....   127.804128: sys_execve(filename: 5bff18, 
argv: 53bb00, envp: 5543a8)
             cat-254     [000] .....   127.809142: sys_execve -> 0x0
From the above, the arm32 output seems correct even though:
# CONFIG_BINFMT_ELF_FDPIC is not set

After some debugging, I realized that arm32 syscall_get_nr() uses abi_syscall 
to get the syscall number and not a register (I guess abi_syscall was set to 
value of R7 before) [1].
So the fact that regs->uregs are memset'ed to 0 is not a problem.

> 
> Best regards.
> ---
> [1] https://elixir.bootlin.com/linux/v5.18/source/arch/arm64/kernel/
> ptrace.c#L1868
> [2] https://elixir.bootlin.com/linux/v5.18/source/arch/arm64/kernel/
> syscall.c#L57


Best regards.
---
[1] https://elixir.bootlin.com/linux/v5.18.8/source/arch/arm/include/asm/
syscall.h#L22