Date: Fri, 16 Oct 2020 21:08:46 +0200 From: Paul Floyd <pjfloyd@wanadoo.fr> To: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: Extra memory mapping seen on freebsd-12 which was not seen in freebsd-11 Message-ID: <C50BCC03-832E-4CB7-8078-0E304705E757@wanadoo.fr> In-Reply-To: <CAB6rxaRMOpx9eotjuNuUX_Ua3UZ-0KCbFdTSdEwXptjbG1%2BGUA@mail.gmail.com> References: <CAB6rxaSNSoCywTy6tx6w%2B%2B-aJhQHb3FJiHoRL0vXs2CJAHe1OA@mail.gmail.com> <9CCF59F6-06F2-4352-94E5-C508E165D0C2@wanadoo.fr> <CAB6rxaRMOpx9eotjuNuUX_Ua3UZ-0KCbFdTSdEwXptjbG1%2BGUA@mail.gmail.com>
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> On 16 Oct 2020, at 07:13, karnajit wangkhem <karnajitw@gmail.com> =
wrote:
>=20
> Thanks for the reply. It helped in my understanding.
>=20
> Below is a sample code
>=20
> #include <stdio.h>
> #include <string.h>
> #include <errno.h>
> #include <sys/mman.h>
>=20
> int main()
> {
> char *str =3D NULL;
> str =3D (char *)mmap((void *)0x7fffdfffe000UL, 0x2000, PROT_READ |
> PROT_WRITE, MAP_FIXED | MAP_ANON, -1, 0);
> if ((void *)str =3D=3D (void *)MAP_FAILED) {
> int err =3D errno;
> printf("mmap failed. err (%s)\n", strerror(err));
> } else {
> memcpy(str, "Hello World", 12);
> printf("str =3D %s\n", str);
> }
>=20
> return 0;
> }
>=20
> Now, the below code under valgrind will give
> - mmap failed. err (Invalid argument)
>=20
> But, if we give control of this segment to the client program
> with VG_(am_change_ownership_v_to_c), then valgrind allows the client =
to do
> the following mmap.
> - str =3D Hello World
>=20
> And, the resultant procstat result looks like this:
> 2382 0x7fbfff000 0x7fc001000 rwx 2 2 1 0 ----- =
df
> 2382 0x7fffdfffe000 0x7fffe0000000 rw- 0 0 0 0 ----- =
--
> <<< Client mmap call
> 2382 0x7fffe0000000 0x7ffffffdf000 --- 0 0 0 0 ----- =
--
> <<< 0x1000 bytes is taken away from the MAP_GUARD area
> 2382 0x7ffffffdf000 0x7ffffffff000 rw- 1 1 1 0 ---D- =
df
> 2382 0x7ffffffff000 0x800000000000 r-x 1 1 104 0 ----- =
ph
>=20
> So, is it right for the application with or without valgrind to cross =
the
> above boundary, If that memory which the application reserved is just =
for
> normal application specific use?
Hi
Obviously threre are some restrictions for the guest application running =
under Valgrind.=20
Valgrind needs its own stack and heap, so the guest can=E2=80=99t mmap =
these regions.
Why do you need to mmap into this region?
A+
Paul
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