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arm64: syscallno is secretly an int, make it official

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The upper 32 bits of the syscallno field in thread_struct are
handled inconsistently, being sometimes zero extended and sometimes
sign-extended.  In fact, only the lower 32 bits seem to have any
real significance for the behaviour of the code: it's been OK to
handle the upper bits inconsistently because they don't matter.

Currently, the only place I can find where those bits are
significant is in calling trace_sys_enter(), which may be
unintentional: for example, if a compat tracer attempts to cancel a
syscall by passing -1 to (COMPAT_)PTRACE_SET_SYSCALL at the
syscall-enter-stop, it will be traced as syscall 4294967295
rather than -1 as might be expected (and as occurs for a native
tracer doing the same thing).  Elsewhere, reads of syscallno cast
it to an int or truncate it.

There's also a conspicuous amount of code and casting to bodge
around the fact that although semantically an int, syscallno is
stored as a u64.

Let's not pretend any more.

In order to preserve the stp x instruction that stores the syscall
number in entry.S, this patch special-cases the layout of struct
pt_regs for big endian so that the newly 32-bit syscallno field
maps onto the low bits of the stored value.  This is not beautiful,
but benchmarking of the getpid syscall on Juno suggests indicates a
minor slowdown if the stp is split into an stp x and stp w.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
This commit is contained in:
Dave Martin
2017-08-01 15:35:53 +01:00
committed by Catalin Marinas
parent aae4e7a8bc
commit 35d0e6fb4d
7 changed files with 32 additions and 25 deletions
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6
arch/arm64/kernel/signal.c
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@@ -387,7 +387,7 @@ static int restore_sigframe(struct pt_regs *regs,
/*
* Avoid sys_rt_sigreturn() restarting.
*/
regs->syscallno = ~0UL;
regs->syscallno = ~0;
err |= !valid_user_regs(&regs->user_regs, current);
if (err == 0)
@@ -673,7 +673,7 @@ static void do_signal(struct pt_regs *regs)
{
unsigned long continue_addr = 0, restart_addr = 0;
int retval = 0;
int syscall = (int)regs->syscallno;
int syscall = regs->syscallno;
struct ksignal ksig;
/*
@@ -687,7 +687,7 @@ static void do_signal(struct pt_regs *regs)
/*
* Avoid additional syscall restarting via ret_to_user.
*/
regs->syscallno = ~0UL;
regs->syscallno = ~0;
/*
* Prepare for system call restart. We do this here so that a