would be nice to explain syscall macros
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The syscall macros are a little dense to decipher. It took me a while to determine how the macro syscall1(int,setuid,uid_t,uid) expanded into the assembly code shown. It might be nice to show the macro, and explain a little about how it gets expanded. Here is the source for the _syscall1 macro
#define _syscall1(type,name,type1,arg1) \
type name(type1 arg1) \
{ \
long __res; \
__asm__ volatile ("int $0x80" \
: "=a" (__res) \
: "0" (__NR_##name),"b" ((long)(arg1))); \
if (__res >= 0) \
return (type) __res; \
errno = -__res; \
return -1; \
}
When expanded, this become the code
int setuid(uid_t uid)
{
long __res;
__asm__ volatile ("int $0x80" \
: "=a" (__res) \
: "0" (__NR_setuid), "b" ((long)(uid)));
if (__res >= 0 )
return (int) __res;
errno = -__res;
return -1;
}
It's pretty easy to see how the cleanup code converts into assembly, but the setup code eluded me until I figured out the following: "=a" (__res) means the result comes back in %eax "0" (__NR_setuid) means put the system call number into %eax on entry "b" ((long)(uid) means put the first argument into %ebx on entry syscallX macros that use additional parameters use %ecx, %edx, %esi, and %edi to hold additional values passed through the call. |
