Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 17 Oct 2017 15:01:34 -0700 | Subject | Re: [PATCH] x86, syscalls: use SYSCALL_DEFINE() macros for sys_modify_ldt() |
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On Tue, Oct 17, 2017 at 12:22 PM, Dave Hansen <dave.hansen@linux.intel.com> wrote: > > We do not have tracepoints for sys_modify_ldt() because we define > it directly instead of using the normal SYSCALL_DEFINEx() macros. > > However, there is a reason sys_modify_ldt() does not use the macros: > it has an 'int' return type instead of 'unsigned long'. This is > a bug, but it's a bug cemented in the ABI. > > What does this mean? If we return -EINVAL from a function that > returns 'int', we have 0x00000000ffffffea in %rax. But, if we > return -EINVAL from a function returning 'unsigned long', we end > up with 0xffffffffffffffea in %rax, which is wrong. > > To work around this and maintain the 'int' behavior while using > the SYSCALL_DEFINEx() macros, so we add a cast to 'unsigned int' > in both implementations of sys_modify_ldt().
Reviewed-by: Andy Lutomirski <luto@kernel.org>
> > Cc: x86@kernel.org > Cc: Andy Lutomirski <luto@kernel.org> > Cc: Brian Gerst <brgerst@gmail.com> > > --- > > b/arch/x86/include/asm/syscalls.h | 2 +- > b/arch/x86/kernel/ldt.c | 16 +++++++++++++--- > b/arch/x86/um/ldt.c | 6 ++++-- > 3 files changed, 18 insertions(+), 6 deletions(-) > > diff -puN arch/x86/include/asm/syscalls.h~x86-syscall-macros-modify_ldt arch/x86/include/asm/syscalls.h > --- a/arch/x86/include/asm/syscalls.h~x86-syscall-macros-modify_ldt 2017-10-17 10:30:53.032718307 -0700 > +++ b/arch/x86/include/asm/syscalls.h 2017-10-17 10:30:53.039718307 -0700 > @@ -21,7 +21,7 @@ asmlinkage long sys_ioperm(unsigned long > asmlinkage long sys_iopl(unsigned int); > > /* kernel/ldt.c */ > -asmlinkage int sys_modify_ldt(int, void __user *, unsigned long); > +asmlinkage long sys_modify_ldt(int, void __user *, unsigned long); > > /* kernel/signal.c */ > asmlinkage long sys_rt_sigreturn(void); > diff -puN arch/x86/kernel/ldt.c~x86-syscall-macros-modify_ldt arch/x86/kernel/ldt.c > --- a/arch/x86/kernel/ldt.c~x86-syscall-macros-modify_ldt 2017-10-17 10:30:53.034718307 -0700 > +++ b/arch/x86/kernel/ldt.c 2017-10-17 12:02:51.137704545 -0700 > @@ -12,6 +12,7 @@ > #include <linux/string.h> > #include <linux/mm.h> > #include <linux/smp.h> > +#include <linux/syscalls.h> > #include <linux/slab.h> > #include <linux/vmalloc.h> > #include <linux/uaccess.h> > @@ -294,8 +295,8 @@ out: > return error; > } > > -asmlinkage int sys_modify_ldt(int func, void __user *ptr, > - unsigned long bytecount) > +SYSCALL_DEFINE3(modify_ldt, int , func , void __user * , ptr , > + unsigned long , bytecount) > { > int ret = -ENOSYS; > > @@ -313,5 +314,14 @@ asmlinkage int sys_modify_ldt(int func, > ret = write_ldt(ptr, bytecount, 0); > break; > } > - return ret; > + /* > + * The SYSCALL_DEFINE() macros give us an 'unsigned long' > + * return type, but tht ABI for sys_modify_ldt() expects > + * 'int'. This cast gives us an int-sized value in %rax > + * for the return code. The 'unsigned' is necessary so > + * the compiler does not try to sign-extend the negative > + * return codes into the high half of the register when > + * taking the value from int->long. > + */ > + return (unsigned int)ret; > } > diff -puN arch/x86/um/ldt.c~x86-syscall-macros-modify_ldt arch/x86/um/ldt.c > --- a/arch/x86/um/ldt.c~x86-syscall-macros-modify_ldt 2017-10-17 10:30:53.036718307 -0700 > +++ b/arch/x86/um/ldt.c 2017-10-17 10:58:58.711714103 -0700 > @@ -369,7 +369,9 @@ void free_ldt(struct mm_context *mm) > mm->arch.ldt.entry_count = 0; > } > > -int sys_modify_ldt(int func, void __user *ptr, unsigned long bytecount) > +SYSCALL_DEFINE3(modify_ldt, int , func , void __user * , ptr , > + unsigned long , bytecount) > { > - return do_modify_ldt_skas(func, ptr, bytecount); > + /* See non-um modify_ldt() for why we do this cast */ > + return (unsigned int)do_modify_ldt_skas(func, ptr, bytecount); > } > _
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