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Re: [PATCH] Add Prefer_MAP_32BIT_EXEC for Silvermont
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: Adhemerval Zanella <adhemerval dot zanella at linaro dot org>
- Cc: GNU C Library <libc-alpha at sourceware dot org>
- Date: Fri, 11 Dec 2015 09:14:19 -0800
- Subject: Re: [PATCH] Add Prefer_MAP_32BIT_EXEC for Silvermont
- Authentication-results: sourceware.org; auth=none
- References: <20151211143706 dot GA7868 at intel dot com> <alpine dot DEB dot 2 dot 10 dot 1512111539300 dot 17023 at digraph dot polyomino dot org dot uk> <CAMe9rOqbqyFw3CMa35vwOEefdFq1xK2Q9hX8GXoGMKVZ-A2y0g at mail dot gmail dot com> <566AF894 dot 4060300 at linaro dot org> <CAMe9rOr-LypZXvq4Y4uwE_JybYoTXctZXMLjo4TH517NnC6omg at mail dot gmail dot com> <566B01BE dot 1070703 at linaro dot org>
On Fri, Dec 11, 2015 at 9:02 AM, Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
>
> On 11-12-2015 14:40, H.J. Lu wrote:
>> On Fri, Dec 11, 2015 at 8:23 AM, Adhemerval Zanella
>> <adhemerval.zanella@linaro.org> wrote:
>>>
>>>
>>> On 11-12-2015 13:59, H.J. Lu wrote:
>>>> On Fri, Dec 11, 2015 at 7:39 AM, Joseph Myers <joseph@codesourcery.com> wrote:
>>>>>> On Fri, 11 Dec 2015, H.J. Lu wrote:
>>>>>>
>>>>>>>> +++ b/sysdeps/unix/sysv/linux/x86_64/64/mmap.c
>>>>>>>> @@ -0,0 +1,49 @@
>>>>>>>> +/* Copyright (C) 2015 Free Software Foundation, Inc.
>>>>>>
>>>>>> All new files should have a descriptive first line before the copyright
>>>>>> notice.
>>>>>>
>>>> Here is the updated patch.
>>>
>>>>
>>>> diff --git a/sysdeps/unix/sysv/linux/x86_64/64/mmap.c b/sysdeps/unix/sysv/linux/x86_64/64/mmap.c
>>>> new file mode 100644
>>>> index 0000000..c34f633
>>>> --- /dev/null
>>>> +++ b/sysdeps/unix/sysv/linux/x86_64/64/mmap.c
>>>
>>>> +
>>>> +__ptr_t
>>>> +__mmap (__ptr_t addr, size_t len, int prot, int flags, int fd, off_t offset)
>>>> +{
>>>> + /* If the Prefer_MAP_32BIT_EXEC bit is set, try to map executable pages
>>>> + with MAP_32BIT first. */
>>>> + if (addr == NULL
>>>> + && (prot & PROT_EXEC) != 0
>>>> + && HAS_ARCH_FEATURE (Prefer_MAP_32BIT_EXEC))
>>>> + {
>>>> + addr = (__ptr_t) INLINE_SYSCALL (mmap, 6, addr, len, prot,
>>>> + flags | MAP_32BIT,
>>>> + fd, offset);
>>>> + if (addr != MAP_FAILED)
>>>> + return addr;
>>>> + }
>>>> + return (__ptr_t) INLINE_SYSCALL (mmap, 6, addr, len, prot, flags,
>>>> + fd, offset);
>>>> +}
>>>> +
>>>
>>> I would advise not add another syscall variant implementation, but rather
>>> work on make a generic implementation with adjustments made by each platform.
>>> Something like
>>>
>>> __ptr_t
>>> __mmap (__ptr_t addr, size_t len, int prot, int flags, int fd, off_t offset)
>>> {
>>> flags = MMAP_ARCH_FLAGS (flags);
>>> return (__ptr_t) INLINE_SYSCALL( mmap, 6, addr, len, prot, flags, fd, offset);
>>> }
>>>
>>> And then define MMAP_ARCH_FLAG for x86 to add the MAP_32BIT when required.
>>>
>>
>> This won't work here since we fallback to the default mmap when
>> MAP_32BIT fails.
>
> So just change the MMAP_ARCH_FLAGS hook to something like:
>
> __ptr_t ret = __mmap_arch (addr, len, prot, flags, fd, offset));
> if (ret != MAP_FAILED)
> ret = INLINE_SYSCALL (mmap, 6, addr, len, prot, flags, fd, offset);
> return ret
I will give it a try.
> The default value for __mmap_arch could be a constant value so compiler
> could remove the the comparison if it is the case.
>
> Another issue is this is basically limiting ALSR really hard on x86_64.
> I also would prefer to make the default to *not* include this flag and
> set the env. variable to actually enable it. If the cpu is slow doing
> what's intended because it is buggy, let it be slow at default. Do not
> break what was intended (full ALSR).
I will discuss it internally.
Thanks.
--
H.J.