Re: git: 6e66030c4c05 - main - linux: implement PTRACE_EVENT_EXEC

From: Konstantin Belousov <kostikbel_at_gmail.com>
Date: Sat, 23 Oct 2021 22:12:34 UTC
On Sat, Oct 23, 2021 at 06:46:34PM +0000, Edward Tomasz Napierala wrote:
> The branch main has been updated by trasz:
> 
> URL: https://cgit.FreeBSD.org/src/commit/?id=6e66030c4c05331f9b0adf87c31f2f233dd3ae1f
> 
> commit 6e66030c4c05331f9b0adf87c31f2f233dd3ae1f
> Author:     Edward Tomasz Napierala <trasz@FreeBSD.org>
> AuthorDate: 2021-10-23 18:13:14 +0000
> Commit:     Edward Tomasz Napierala <trasz@FreeBSD.org>
> CommitDate: 2021-10-23 18:46:26 +0000
> 
>     linux: implement PTRACE_EVENT_EXEC
>     
>     This fixes strace(1) from Ubuntu Focal.
>     
>     Reviewed By:    jhb
>     Sponsored By:   EPSRC
>     Differential Revision:  https://reviews.freebsd.org/D32367
> ---
>  sys/amd64/linux/linux_ptrace.c |  9 +++++++--
>  sys/kern/subr_syscall.c        | 12 ++++++++++++
>  2 files changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/sys/amd64/linux/linux_ptrace.c b/sys/amd64/linux/linux_ptrace.c
> index 37f136856ef0..d7dffc92dd11 100644
> --- a/sys/amd64/linux/linux_ptrace.c
> +++ b/sys/amd64/linux/linux_ptrace.c
> @@ -71,6 +71,7 @@ __FBSDID("$FreeBSD$");
>  #define	LINUX_PTRACE_SEIZE		0x4206
>  #define	LINUX_PTRACE_GET_SYSCALL_INFO	0x420e
>  
> +#define	LINUX_PTRACE_EVENT_EXEC		4
>  #define	LINUX_PTRACE_EVENT_EXIT		6
>  
>  #define	LINUX_PTRACE_O_TRACESYSGOOD	1
> @@ -151,8 +152,12 @@ linux_ptrace_status(struct thread *td, pid_t pid, int status)
>  	    lwpinfo.pl_flags & PL_FLAG_SCE)
>  		status |= (LINUX_SIGTRAP | 0x80) << 8;
>  	if ((pem->ptrace_flags & LINUX_PTRACE_O_TRACESYSGOOD) &&
> -	    lwpinfo.pl_flags & PL_FLAG_SCX)
> -		status |= (LINUX_SIGTRAP | 0x80) << 8;
> +	    lwpinfo.pl_flags & PL_FLAG_SCX) {
> +		if (lwpinfo.pl_flags & PL_FLAG_EXEC)
> +			status |= (LINUX_SIGTRAP | LINUX_PTRACE_EVENT_EXEC << 8) << 8;
> +		else
> +			status |= (LINUX_SIGTRAP | 0x80) << 8;
> +	}
>  	if ((pem->ptrace_flags & LINUX_PTRACE_O_TRACEEXIT) &&
>  	    lwpinfo.pl_flags & PL_FLAG_EXITED)
>  		status |= (LINUX_SIGTRAP | LINUX_PTRACE_EVENT_EXIT << 8) << 8;
> diff --git a/sys/kern/subr_syscall.c b/sys/kern/subr_syscall.c
> index ada2053bc87c..fab67a68b0a3 100644
> --- a/sys/kern/subr_syscall.c
> +++ b/sys/kern/subr_syscall.c
> @@ -254,6 +254,18 @@ syscallret(struct thread *td)
>  	if (__predict_false(traced ||
>  	    (td->td_dbgflags & (TDB_EXEC | TDB_FORK)) != 0)) {
>  		PROC_LOCK(p);
> +		/*
> +		 * Linux debuggers expect an additional stop for exec,
> +		 * between the usual syscall entry and exit.  Raise
> +		 * the exec event now and then clear TDB_EXEC so that
> +		 * the next stop is reported as a syscall exit by
> +		 * linux_ptrace_status().
> +		 */
> +		if ((td->td_dbgflags & TDB_EXEC) != 0 &&
> +		    SV_PROC_ABI(td->td_proc) == SV_ABI_LINUX) {
> +			ptracestop(td, SIGTRAP, NULL);
> +			td->td_dbgflags &= ~TDB_EXEC;
> +		}
>  		/*
>  		 * If tracing the execed process, trap to the debugger
>  		 * so that breakpoints can be set before the program
You committed before we finished the discussion.

In fact I do not think it is right.  Now it generates spurious stop for
FreeBSD native debugger, and probably de-synchronize SCX/SCE pairing for
it as well.

In other words, you need to generate this stop based on ABI of the debugger,
not debuggeeeeee.