Re: [PATCH] powerpc/mm: restore SEGV_ACCERR for segv on mapped region

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Sun, 2017-12-31 at 21:24 -0800, John Sperbeck wrote:
> A recent refactoring of the powerpc page fault handler caused
> access to protected memory regions to indicate SEGV_MAPERR instead
> of the traditional SEGV_ACCERR in the si_code field of a user-space
> signal handler.  This can confuse debug libraries that temporarily
> change the protection of memory regions, and expect to use
> SEGV_ACCERR as an indication to restore access to a region.
> 
> This change restores the previous behavior.  The following program
> exhibits the issue:

Good catch !

Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> 
>     $ ./repro read  || echo "FAILED"
>     $ ./repro write || echo "FAILED"
>     $ ./repro exec  || echo "FAILED"
> 
>     #include <stdio.h>
>     #include <stdlib.h>
>     #include <string.h>
>     #include <unistd.h>
>     #include <signal.h>
>     #include <sys/mman.h>
>     #include <assert.h>
> 
>     static void segv_handler(int n, siginfo_t *info, void *arg) {
>             _exit(info->si_code == SEGV_ACCERR ? 0 : 1);
>     }
> 
>     int main(int argc, char **argv)
>     {
>             void *p = NULL;
>             struct sigaction act = {
>                     .sa_sigaction = segv_handler,
>                     .sa_flags = SA_SIGINFO,
>             };
> 
>             assert(argc == 2);
>             p = mmap(NULL, getpagesize(),
>                     (strcmp(argv[1], "write") == 0) ? PROT_READ : 0,
>                     MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
>             assert(p != MAP_FAILED);
> 
>             assert(sigaction(SIGSEGV, &act, NULL) == 0);
>             if (strcmp(argv[1], "read") == 0)
>                     printf("%c", *(unsigned char *)p);
>             else if (strcmp(argv[1], "write") == 0)
>                     *(unsigned char *)p = 0;
>             else if (strcmp(argv[1], "exec") == 0)
>                     ((void (*)(void))p)();
>             return 1;  /* failed to generate SEGV */
>     }
> 
> Signed-off-by: John Sperbeck <jsperbeck@google.com>
> ---
>  arch/powerpc/mm/fault.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> index 4797d08581ce..6e1e39035380 100644
> --- a/arch/powerpc/mm/fault.c
> +++ b/arch/powerpc/mm/fault.c
> @@ -145,6 +145,11 @@ static noinline int bad_area(struct pt_regs *regs, unsigned long address)
>  	return __bad_area(regs, address, SEGV_MAPERR);
>  }
>  
> +static noinline int bad_access(struct pt_regs *regs, unsigned long address)
> +{
> +	return __bad_area(regs, address, SEGV_ACCERR);
> +}
> +
>  static int do_sigbus(struct pt_regs *regs, unsigned long address,
>  		     unsigned int fault)
>  {
> @@ -490,7 +495,7 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
>  
>  good_area:
>  	if (unlikely(access_error(is_write, is_exec, vma)))
> -		return bad_area(regs, address);
> +		return bad_access(regs, address);
>  
>  	/*
>  	 * If for any reason at all we couldn't handle the fault,