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Subject[PATCH 5.0 164/238] powerpc/ptrace: Simplify vr_get/set() to avoid GCC warning
Date
5.0-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Michael Ellerman <mpe@ellerman.id.au>

commit ca6d5149d2ad0a8d2f9c28cbe379802260a0a5e0 upstream.

GCC 8 warns about the logic in vr_get/set(), which with -Werror breaks
the build:

In function ‘user_regset_copyin’,
inlined from ‘vr_set’ at arch/powerpc/kernel/ptrace.c:628:9:
include/linux/regset.h:295:4: error: ‘memcpy’ offset [-527, -529] is
out of the bounds [0, 16] of object ‘vrsave’ with type ‘union
<anonymous>’ [-Werror=array-bounds]
arch/powerpc/kernel/ptrace.c: In function ‘vr_set’:
arch/powerpc/kernel/ptrace.c:623:5: note: ‘vrsave’ declared here
} vrsave;

This has been identified as a regression in GCC, see GCC bug 88273.

However we can avoid the warning and also simplify the logic and make
it more robust.

Currently we pass -1 as end_pos to user_regset_copyout(). This says
"copy up to the end of the regset".

The definition of the regset is:
[REGSET_VMX] = {
.core_note_type = NT_PPC_VMX, .n = 34,
.size = sizeof(vector128), .align = sizeof(vector128),
.active = vr_active, .get = vr_get, .set = vr_set
},

The end is calculated as (n * size), ie. 34 * sizeof(vector128).

In vr_get/set() we pass start_pos as 33 * sizeof(vector128), meaning
we can copy up to sizeof(vector128) into/out-of vrsave.

The on-stack vrsave is defined as:
union {
elf_vrreg_t reg;
u32 word;
} vrsave;

And elf_vrreg_t is:
typedef __vector128 elf_vrreg_t;

So there is no bug, but we rely on all those sizes lining up,
otherwise we would have a kernel stack exposure/overwrite on our
hands.

Rather than relying on that we can pass an explict end_pos based on
the sizeof(vrsave). The result should be exactly the same but it's
more obviously not over-reading/writing the stack and it avoids the
compiler warning.

Reported-by: Meelis Roos <mroos@linux.ee>
Reported-by: Mathieu Malaterre <malat@debian.org>
Cc: stable@vger.kernel.org
Tested-by: Mathieu Malaterre <malat@debian.org>
Tested-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/powerpc/kernel/ptrace.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)

--- a/arch/powerpc/kernel/ptrace.c
+++ b/arch/powerpc/kernel/ptrace.c
@@ -561,6 +561,7 @@ static int vr_get(struct task_struct *ta
/*
* Copy out only the low-order word of vrsave.
*/
+ int start, end;
union {
elf_vrreg_t reg;
u32 word;
@@ -569,8 +570,10 @@ static int vr_get(struct task_struct *ta

vrsave.word = target->thread.vrsave;

+ start = 33 * sizeof(vector128);
+ end = start + sizeof(vrsave);
ret = user_regset_copyout(&pos, &count, &kbuf, &ubuf, &vrsave,
- 33 * sizeof(vector128), -1);
+ start, end);
}

return ret;
@@ -608,6 +611,7 @@ static int vr_set(struct task_struct *ta
/*
* We use only the first word of vrsave.
*/
+ int start, end;
union {
elf_vrreg_t reg;
u32 word;
@@ -616,8 +620,10 @@ static int vr_set(struct task_struct *ta

vrsave.word = target->thread.vrsave;

+ start = 33 * sizeof(vector128);
+ end = start + sizeof(vrsave);
ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, &vrsave,
- 33 * sizeof(vector128), -1);
+ start, end);
if (!ret)
target->thread.vrsave = vrsave.word;
}

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