On Wed, Mar 28, 2018 at 2:15 AM, Burton, Ross <ross.bur...@intel.com> wrote:
> With this patch I was getting occasional failures of the pseudo-using
> bitbake-worker, so its not quite ready, but Peter is working on a
> better form anyway.
The "better form" seems to have been committed to the pseudo master
branch. Very interesting...
+long
+syscall(long number, ...) {
+ /* In a fit of optimism, I imagine that if we didn't get at least 7
+ * arguments, reading past the ones we did get will read into this
+ * space and maybe not clash with or overlap with any later-declared
+ * values. This isn't really a guarantee, and is probably just
+ * superstition.
+ */
+ unsigned long long padding[7];
+ (void) padding;
Arguments passed by the caller will be put on the stack before any
stack frame is created by the callee. You can argue about which way a
stack grows (up or down) but however you define it, reading past the
end of the arguments passed on the stack by the caller is never going
to read into the stack frame created by the callee, so this can't have
the intended affect.
Also... any compiler from at least the past 20 years or so is going to
optimise away unused variables, so this does precisely nothing anyway.
+ /* gcc magic to attempt to just pass these args to syscall. we have to
+ * guess about the number of args; the docs discuss calling conventions
+ * up to 7, so let's try that?
+ */
+ void *res = __builtin_apply((void (*)()) real_syscall,
__builtin_apply_args(), sizeof(long long) * 7);
+ __builtin_return(res);
This is probably going to work, but if the goal is to avoid reading
more from the stack than the generic C code would do, it doesn't
succeed. The "size" parameter to __builtin_apply() seems to simply
specify how much argument data to copy from the stack frame passed by
the caller. Setting it to sizeof(long long) * 7 is safe (ie it will
copy at least enough data from the stack frame passed by the caller,
never less) as it covers both the corner cases where registers are
long long (such as x32) and where _no_ arguments are passed in
registers and everything needs to be copied from the stack. However,
on 32bit targets (where registers are smaller than long long) and on
any target where _some_ arguments are passed via registers, it will
cause more data to be read from the stack than the generic C code
would.
e.g. on 32bit ARM where the first 4 integer arguments are passed via
registers, the optimum value for __builtin_apply() "size" in order to
pass through 1 syscall number and 6 additional register sized
arguments would be sizeof(long) * 3.
A simple test built for 32bit ARM seems to confirm that. The generic
code unconditionally reads 12 bytes from the stack frame passed by the
caller. The code now in pseudo master unconditionally reads 56 bytes.
$ cat tst.c
#include <stdarg.h>
extern int real_syscall();
typedef long syscall_arg_t; /* fixme: wrong for x32 */
int wrapper_generic (long int n, ...)
{
va_list ap;
syscall_arg_t a,b,c,d,e,f;
va_start(ap, n);
a=va_arg(ap, syscall_arg_t);
b=va_arg(ap, syscall_arg_t);
c=va_arg(ap, syscall_arg_t);
d=va_arg(ap, syscall_arg_t);
e=va_arg(ap, syscall_arg_t);
f=va_arg(ap, syscall_arg_t);
va_end(ap);
return real_syscall(n,a,b,c,d,e,f);
}
int wrapper_gcc_specific (long int n, ...)
{
void *res = __builtin_apply((void (*)()) real_syscall,
__builtin_apply_args(), sizeof(long long) * 7);
__builtin_return(res);
}
$ arm-linux-gnueabi-objdump -d tst.o
tst.o: file format elf32-littlearm
Disassembly of section .text:
00000000 <wrapper_generic>:
0: e92d000f push {r0, r1, r2, r3}
4: e92d407f push {r0, r1, r2, r3, r4, r5, r6, lr}
8: e28d0020 add r0, sp, #32
c: e59d3038 ldr r3, [sp, #56] ; 0x38
10: e28d2024 add r2, sp, #36 ; 0x24
14: e58d2014 str r2, [sp, #20]
18: e58d3008 str r3, [sp, #8]
1c: e59d3034 ldr r3, [sp, #52] ; 0x34
20: e58d3004 str r3, [sp, #4]
24: e59d3030 ldr r3, [sp, #48] ; 0x30
28: e58d3000 str r3, [sp]
2c: e890000f ldm r0, {r0, r1, r2, r3}
30: ebfffffe bl 0 <real_syscall>
34: e28dd01c add sp, sp, #28
38: e49de004 pop {lr} ; (ldr lr, [sp], #4)
3c: e28dd010 add sp, sp, #16
40: e12fff1e bx lr
00000044 <wrapper_gcc_specific>:
44: e92d000f push {r0, r1, r2, r3}
48: e92d4830 push {r4, r5, fp, lr}
4c: e28db00c add fp, sp, #12
50: e24b400c sub r4, fp, #12
54: e28bc014 add ip, fp, #20
58: e24dd028 sub sp, sp, #40 ; 0x28
5c: e50b0020 str r0, [fp, #-32] ; 0xffffffe0
60: e1a0500d mov r5, sp
64: e50b101c str r1, [fp, #-28] ; 0xffffffe4
68: e24dd040 sub sp, sp, #64 ; 0x40
6c: e50b2018 str r2, [fp, #-24] ; 0xffffffe8
70: e1a0e00d mov lr, sp
74: e50b3014 str r3, [fp, #-20] ; 0xffffffec
78: e524c018 str ip, [r4, #-24]! ; 0xffffffe8
7c: e8bc000f ldm ip!, {r0, r1, r2, r3}
80: e8ae000f stmia lr!, {r0, r1, r2, r3}
84: e8bc000f ldm ip!, {r0, r1, r2, r3}
88: e8ae000f stmia lr!, {r0, r1, r2, r3}
8c: e8bc000f ldm ip!, {r0, r1, r2, r3}
90: e8ae000f stmia lr!, {r0, r1, r2, r3}
94: e89c0003 ldm ip, {r0, r1}
98: e88e0003 stm lr, {r0, r1}
9c: e994000f ldmib r4, {r0, r1, r2, r3}
a0: e24b4034 sub r4, fp, #52 ; 0x34
a4: ebfffffe bl 0 <real_syscall>
a8: e884000f stm r4, {r0, r1, r2, r3}
ac: e1a0d005 mov sp, r5
b0: e894000f ldm r4, {r0, r1, r2, r3}
b4: e24bd00c sub sp, fp, #12
b8: e8bd4830 pop {r4, r5, fp, lr}
bc: e28dd010 add sp, sp, #16
c0: e12fff1e bx lr
(Note in the code was compiled with -mfloat-abi=soft to avoid
__builtin_apply() needing to save and restore all floating point
registers - which doesn't affect the amount of data read from the
stack, but makes the assembler more than twice as long...).
--
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