Don't let cpu_set_syscall_retval() clobber exec_setregs().

Upon successful completion, the execve() system call invokes
exec_setregs() to initialize the registers of the initial thread of the
newly executed process. What is weird is that when execve() returns, it
still goes through the normal system call return path, clobbering the
registers with the system call's return value (td->td_retval).

Though this doesn't seem to be problematic for x86 most of the times (as
the value of eax/rax doesn't matter upon startup), this can be pretty
frustrating for architectures where function argument and return
registers overlap (e.g., ARM). On these systems, exec_setregs() also
needs to initialize td_retval.

Even worse are architectures where cpu_set_syscall_retval() sets
registers to values not derived from td_retval. On these architectures,
there is no way cpu_set_syscall_retval() can set registers to the way it
wants them to be upon the start of execution.

To get rid of this madness, let sys_execve() return EJUSTRETURN. This
will cause cpu_set_syscall_retval() to leave registers intact. This
makes process execution easier to understand. It also eliminates the
difference between execution of the initial process and successive ones.
The initial call to sys_execve() is not performed through a system call
context.

Reviewed by:	kib, jhibbits
Differential Revision:	https://reviews.freebsd.org/D13180
This commit is contained in:
Ed Schouten 2017-11-24 07:35:08 +00:00
parent d927d443e1
commit 814629dd64
Notes: svn2git 2020-12-20 02:59:44 +00:00
svn path=/head/; revision=326145
13 changed files with 16 additions and 45 deletions

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@ -604,7 +604,6 @@ exec_setregs(struct thread *td, struct image_params *imgp, u_long stack)
regs->tf_fs = _ufssel;
regs->tf_gs = _ugssel;
regs->tf_flags = TF_HASSEGS;
td->td_retval[1] = 0;
/*
* Reset the hardware debug registers if they were in use.

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@ -967,5 +967,4 @@ ia32_setregs(struct thread *td, struct image_params *imgp, u_long stack)
/* Return via doreti so that we can change to a different %cs */
set_pcb_flags(pcb, PCB_32BIT | PCB_FULL_IRET);
td->td_retval[1] = 0;
}

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@ -832,7 +832,6 @@ exec_linux_setregs(struct thread *td, struct image_params *imgp, u_long stack)
/* Do full restore on return so that we can change to a different %cs */
set_pcb_flags(pcb, PCB_32BIT | PCB_FULL_IRET);
td->td_retval[1] = 0;
}
/*

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@ -61,7 +61,7 @@ cloudabi32_proc_setregs(struct thread *td, struct image_params *imgp,
* tpidrurw to the TCB.
*/
regs = td->td_frame;
regs->tf_r0 = td->td_retval[0] =
regs->tf_r0 =
stack + roundup(sizeof(cloudabi32_tcb_t), sizeof(register_t));
(void)cpu_set_user_tls(td, (void *)stack);
}

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@ -311,12 +311,7 @@ exec_setregs(struct thread *td, struct image_params *imgp, u_long stack)
memset(tf, 0, sizeof(struct trapframe));
/*
* We need to set x0 for init as it doesn't call
* cpu_set_syscall_retval to copy the value. We also
* need to set td_retval for the cases where we do.
*/
tf->tf_x[0] = td->td_retval[0] = stack;
tf->tf_x[0] = stack;
tf->tf_sp = STACKALIGN(stack);
tf->tf_lr = imgp->entry_addr;
tf->tf_elr = imgp->entry_addr;

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@ -61,7 +61,7 @@ cloudabi64_proc_setregs(struct thread *td, struct image_params *imgp,
* tpidr_el0 to the TCB.
*/
regs = td->td_frame;
regs->tf_x[0] = td->td_retval[0] =
regs->tf_x[0] =
stack + roundup(sizeof(cloudabi64_tcb_t), sizeof(register_t));
(void)cpu_set_user_tls(td, (void *)stack);
}

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@ -186,7 +186,7 @@ linux_common_execve(struct thread *td, struct image_args *eargs)
error = kern_execve(td, eargs, NULL);
post_execve(td, error, oldvmspace);
if (error != 0)
if (error != EJUSTRETURN)
return (error);
/*
@ -213,7 +213,7 @@ linux_common_execve(struct thread *td, struct image_args *eargs)
free(em, M_TEMP);
free(pem, M_LINUX);
}
return (0);
return (EJUSTRETURN);
}
void

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@ -1126,6 +1126,7 @@ exec_setregs(struct thread *td, struct image_params *imgp, u_long stack)
set_fsbase(td, 0);
set_gsbase(td, 0);
/* Make sure edx is 0x0 on entry. Linux binaries depend on it. */
bzero((char *)regs, sizeof(struct trapframe));
regs->tf_eip = imgp->entry_addr;
regs->tf_esp = stack;
@ -1168,13 +1169,6 @@ exec_setregs(struct thread *td, struct image_params *imgp, u_long stack)
* clean FP state if it uses the FPU again.
*/
fpstate_drop(td);
/*
* XXX - Linux emulator
* Make sure sure edx is 0x0 on entry. Linux binaries depend
* on it.
*/
td->td_retval[1] = 0;
}
void

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@ -797,7 +797,7 @@ start_init(void *dummy)
* Otherwise, return via fork_trampoline() all the way
* to user mode as init!
*/
if ((error = sys_execve(td, &args)) == 0) {
if ((error = sys_execve(td, &args)) == EJUSTRETURN) {
mtx_unlock(&Giant);
return;
}

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@ -318,7 +318,7 @@ post_execve(struct thread *td, int error, struct vmspace *oldvmspace)
* If success, we upgrade to SINGLE_EXIT state to
* force other threads to suicide.
*/
if (error == 0)
if (error == EJUSTRETURN)
thread_single(p, SINGLE_EXIT);
else
thread_single_end(p, SINGLE_BOUNDARY);
@ -962,7 +962,13 @@ do_execve(struct thread *td, struct image_args *args, struct mac *mac_p)
ktrprocctor(p);
#endif
return (error);
/*
* We don't want cpu_set_syscall_retval() to overwrite any of
* the register values put in place by exec_setregs().
* Implementations of cpu_set_syscall_retval() will leave
* registers unmodified when returning EJUSTRETURN.
*/
return (error == 0 ? EJUSTRETURN : error);
}
int

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@ -520,22 +520,11 @@ exec_setregs(struct thread *td, struct image_params *imgp, u_long stack)
* - ps_strings is a NetBSD extention, and will be
* ignored by executables which are strictly
* compliant with the SVR4 ABI.
*
* XXX We have to set both regs and retval here due to different
* XXX calling convention in trap.c and init_main.c.
*/
/* Collect argc from the user stack */
argc = fuword((void *)stack);
/*
* XXX PG: these get overwritten in the syscall return code.
* execve() should return EJUSTRETURN, like it does on NetBSD.
* Emulate by setting the syscall return value cells. The
* registers still have to be set for init's fork trampoline.
*/
td->td_retval[0] = argc;
td->td_retval[1] = stack + sizeof(register_t);
tf->fixreg[3] = argc;
tf->fixreg[4] = stack + sizeof(register_t);
tf->fixreg[5] = stack + (2 + argc)*sizeof(register_t);
@ -572,8 +561,6 @@ ppc32_setregs(struct thread *td, struct image_params *imgp, u_long stack)
argc = fuword32((void *)stack);
td->td_retval[0] = argc;
td->td_retval[1] = stack + sizeof(uint32_t);
tf->fixreg[3] = argc;
tf->fixreg[4] = stack + sizeof(uint32_t);
tf->fixreg[5] = stack + (2 + argc)*sizeof(uint32_t);

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@ -279,12 +279,7 @@ exec_setregs(struct thread *td, struct image_params *imgp, u_long stack)
memset(tf, 0, sizeof(struct trapframe));
/*
* We need to set a0 for init as it doesn't call
* cpu_set_syscall_retval to copy the value. We also
* need to set td_retval for the cases where we do.
*/
tf->tf_a[0] = td->td_retval[0] = stack;
tf->tf_a[0] = stack;
tf->tf_sp = STACKALIGN(stack);
tf->tf_ra = imgp->entry_addr;
tf->tf_sepc = imgp->entry_addr;

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@ -1009,9 +1009,6 @@ exec_setregs(struct thread *td, struct image_params *imgp, u_long stack)
* header, it turns out that just always using TSO performs best.
*/
tf->tf_tstate = TSTATE_IE | TSTATE_PEF | TSTATE_MM_TSO;
td->td_retval[0] = tf->tf_out[0];
td->td_retval[1] = tf->tf_out[1];
}
int