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SubjectRe: [PATCH v2] powerpc/uaccess: Use flexible addressing with __put_user()/__get_user()
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Le 30/06/2020 à 23:18, Segher Boessenkool a écrit :
> Hi again,
>
> Thanks for your work so far!
>
> On Tue, Jun 30, 2020 at 06:53:39PM +0000, Christophe Leroy wrote:
>> On 06/30/2020 04:33 PM, Segher Boessenkool wrote:
>>>>> + make -s CC=powerpc64-linux-gnu-gcc -j 160
>>>>> In file included from /linux/include/linux/uaccess.h:11:0,
>>>>> from /linux/include/linux/sched/task.h:11,
>>>>> from /linux/include/linux/sched/signal.h:9,
>>>>> from /linux/include/linux/rcuwait.h:6,
>>>>> from /linux/include/linux/percpu-rwsem.h:7,
>>>>> from /linux/include/linux/fs.h:33,
>>>>> from /linux/include/linux/huge_mm.h:8,
>>>>> from /linux/include/linux/mm.h:675,
>>>>> from /linux/arch/powerpc/kernel/signal_32.c:17:
>>>>> /linux/arch/powerpc/kernel/signal_32.c: In function
>>>>> 'save_user_regs.isra.14.constprop':
>>>>> /linux/arch/powerpc/include/asm/uaccess.h:161:2: error: 'asm' operand has
>>>>> impossible constraints
>>>>> __asm__ __volatile__( \
>>>>> ^
>>>>> /linux/arch/powerpc/include/asm/uaccess.h:197:12: note: in expansion of
>>>>> macro '__put_user_asm'
>>>>> case 4: __put_user_asm(x, ptr, retval, "stw"); break; \
>>>>> ^
>>>>> /linux/arch/powerpc/include/asm/uaccess.h:206:2: note: in expansion of
>>>>> macro '__put_user_size_allowed'
>>>>> __put_user_size_allowed(x, ptr, size, retval); \
>>>>> ^
>>>>> /linux/arch/powerpc/include/asm/uaccess.h:220:2: note: in expansion of
>>>>> macro '__put_user_size'
>>>>> __put_user_size(__pu_val, __pu_addr, __pu_size, __pu_err); \
>>>>> ^
>>>>> /linux/arch/powerpc/include/asm/uaccess.h:96:2: note: in expansion of
>>>>> macro '__put_user_nocheck'
>>>>> __put_user_nocheck((__typeof__(*(ptr)))(x), (ptr), sizeof(*(ptr)))
>>>>> ^
>>>>> /linux/arch/powerpc/kernel/signal_32.c:120:7: note: in expansion of macro
>>>>> '__put_user'
>>>>> if (__put_user((unsigned int)gregs[i], &frame->mc_gregs[i]))
>>>>> ^
>>>
>>> Can we see what that was after the macro jungle? Like, the actual
>>> preprocessed code?
>>
>> Sorry for previous misunderstanding
>>
>> Here is the code:
>>
>> #define __put_user_asm(x, addr, err, op) \
>> __asm__ __volatile__( \
>> "1: " op "%U2%X2 %1,%2 # put_user\n" \
>> "2:\n" \
>> ".section .fixup,\"ax\"\n" \
>> "3: li %0,%3\n" \
>> " b 2b\n" \
>> ".previous\n" \
>> EX_TABLE(1b, 3b) \
>> : "=r" (err) \
>> : "r" (x), "m<>" (*addr), "i" (-EFAULT), "0" (err))
>
> Yeah I don't see it. I'll have to look at compiler debug dumps, but I
> don't have any working 4.9 around, and I cannot reproduce this with
> either older or newer compilers.

I reproduced it with 4.8.5

>
> It is complainig that constrain_operands just does not work *at all* on
> this "m<>" constraint apparently, which doesn't make much sense.
>

Here is a small reproducer:

#include <linux/elf.h>
#include <linux/ptrace.h>
#include <linux/uaccess.h>

struct mcontext {
elf_gregset_t32 mc_gregs;
elf_fpregset_t mc_fregs;
unsigned int mc_pad[2];
elf_vrregset_t32 mc_vregs __attribute__((__aligned__(16)));
elf_vsrreghalf_t32 mc_vsregs __attribute__((__aligned__(16)));
};

int save_general_regs(struct pt_regs *regs, struct mcontext __user *frame)
{
elf_greg_t64 *gregs = (elf_greg_t64 *)regs;
int i;

for (i = 0; i <= PT_RESULT; i ++) {
if (i == 14)
i = 32;
if (__put_user((unsigned int)gregs[i], &frame->mc_gregs[i]))
return -EFAULT;
}
return 0;
}


If you remove the "if i == 14 ..." you get no failure.

Preprocessor result:

int save_general_regs(struct pt_regs *regs, struct mcontext *frame)
{
elf_greg_t64 *gregs = (elf_greg_t64 *)regs;
int i;

for (i = 0; i <= 43; i ++) {
if (i == 14)
i = 32;
if (({ long __pu_err; __typeof__(*((&frame->mc_gregs[i]))) *__pu_addr
= ((&frame->mc_gregs[i])); __typeof__(*((&frame->mc_gregs[i]))) __pu_val
= ((__typeof__(*(&frame->mc_gregs[i])))((unsigned int)gregs[i]));
__typeof__(sizeof(*(&frame->mc_gregs[i]))) __pu_size =
(sizeof(*(&frame->mc_gregs[i]))); if (!(((unsigned long)__pu_addr) >=
0x8000000000000000ul)) might_fault(); (void)0; do {
allow_write_to_user(__pu_addr, __pu_size); do { __pu_err = 0; switch
(__pu_size) { case 1: __asm__ __volatile__( "1: " "stb" "%U2%X2 %1,%2 #
put_user\n" "2:\n" ".section .fixup,\"ax\"\n" "3: li %0,%3\n" " b 2b\n"
".previous\n" ".section __ex_table,\"a\";" " " ".balign 4;" " " ".long
(1b) - . ;" " " ".long (3b) - . ;" " " ".previous" " " : "=r" (__pu_err)
: "r" (__pu_val), "m<>" (*__pu_addr), "i" (-14), "0" (__pu_err)); break;
case 2: __asm__ __volatile__( "1: " "sth" "%U2%X2 %1,%2 # put_user\n"
"2:\n" ".section .fixup,\"ax\"\n" "3: li %0,%3\n" " b 2b\n"
".previous\n" ".section __ex_table,\"a\";" " " ".balign 4;" " " ".long
(1b) - . ;" " " ".long (3b) - . ;" " " ".previous" " " : "=r" (__pu_err)
: "r" (__pu_val), "m<>" (*__pu_addr), "i" (-14), "0" (__pu_err)); break;
case 4: __asm__ __volatile__( "1: " "stw" "%U2%X2 %1,%2 # put_user\n"
"2:\n" ".section .fixup,\"ax\"\n" "3: li %0,%3\n" " b 2b\n"
".previous\n" ".section __ex_table,\"a\";" " " ".balign 4;" " " ".long
(1b) - . ;" " " ".long (3b) - . ;" " " ".previous" " " : "=r" (__pu_err)
: "r" (__pu_val), "m<>" (*__pu_addr), "i" (-14), "0" (__pu_err)); break;
case 8: __asm__ __volatile__( "1: " "std" "%U2%X2 %1,%2 # put_user\n"
"2:\n" ".section .fixup,\"ax\"\n" "3: li %0,%3\n" " b 2b\n"
".previous\n" ".section __ex_table,\"a\";" " " ".balign 4;" " " ".long
(1b) - . ;" " " ".long (3b) - . ;" " " ".previous" " " : "=r" (__pu_err)
: "r" (__pu_val), "m<>" (*__pu_addr), "i" (-14), "0" (__pu_err)); break;
default: __put_user_bad(); } } while (0);
prevent_write_to_user(__pu_addr, __pu_size); } while (0); __pu_err; }))
return -14;
}
return 0;
}


Christophe

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