Messages in this thread Patch in this message | | | Subject | [PATCH] x86, syscalls: use SYSCALL_DEFINE() macros for sys_modify_ldt() | From | Dave Hansen <> | Date | Tue, 17 Oct 2017 12:22:31 -0700 |
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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().
Cc: x86@kernel.org Cc: Andy Lutomirski <luto@kernel.org> Cc: Brian Gerst <brgerst@gmail.com>
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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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