arm: SIGBUS on unsupported ARMv6 unaligned accesses
PATCH 1/1 arm: SIGBUS on unsupported ARMv6 unaligned accesses
This patch changes the default behaviour for userspace alignment faults on v6 from silent fixup to SIGBUS. This only affects code that violates the v6 unaligned access model - bad assembly/JIT code or high-level language code in violation of the relevant language spec - which should be corrected rather than unwittingly relying on performance-degrading fixups. Fixup behaviour can still be controlled from boot parameters or at runtime for compatibility with existing incorrect software.
There will be no impact for v5 and earlier.
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This is very much the hard-line approach to the issue, but userspace really shouldn't be depending on the ability to execute architecturally invalid code; ultimately this is an ABI that deserves to be broken. The only trouble with going in softly and just turning on the warning alone is that it should already be rare enough to be easily missed. "Break everything AND scream about it" as the default isn't going to make any friends, but does make bad code as visible as possible.
Anyone is of course free to turn warnings off, and/or fixups back on - defaulting to UM_SIGNAL on UM_IGNORE is to ensure fixup has to be explicitly requested, by which point they should be well aware of what they're doing and why.
arch/arm/mm/alignment.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)
arch/arm/mm/alignment.c +3 -3
@@ -106,10 +106,10 @@ static int safe_usermode(int new_usermode, bool warn)* making any progress. */ if (cpu_is_v6_unaligned () && !(new_usermode & (UM_FIXUP |UM_SIGNAL ))) { - new_usermode |=UM_FIXUP ; + new_usermode |=UM_SIGNAL ;if (warn) -printk (KERN_WARNING "alignment: ignoring faults is unsafe on this CPU. Defaulting to fixup mode.\n" ); +printk (KERN_WARNING "alignment: ignoring faults is unsafe on this CPU. Defaulting to signal mode.\n" ); }return new_usermode;
@@ -971,7 +971,7 @@ static int __init alignment_init(void) cr_alignment &= ~CR_A ; cr_no_alignment &= ~CR_A ;set_cr (cr_alignment); - ai_usermode = safe_usermode(ai_usermode, false); + ai_usermode = safe_usermode(ai_usermode | UM_WARN, false); }#endif
Signature · 1 line
1.7.9.5
PATCH 1/1 arm: SIGBUS on unsupported ARMv6 unaligned accesses
Quoted 11 lines · Robin Murphy
This patch changes the default behaviour for userspace alignment faults on v6 from silent fixup to SIGBUS. This only affects code that violates the v6 unaligned access model - bad assembly/JIT code or high-level language code in violation of the relevant language spec - which should be corrected rather than unwittingly relying on performance-degrading fixups. Fixup behaviour can still be controlled from boot parameters or at runtime for compatibility with existing incorrect software.
There will be no impact for v5 and earlier.
Signed-off-by Robin Murphy <[email protected]>
People sholdn't be relying on transparent alignment fixups any more, and anyone doing so is already doing something wrong and would benefit from knowing about it.
Since nobody else has raised concerns, I would vote for going ahead with this.
People running OABI userspace on v6 might fall foul of this, but I think requiring them to pass an alignment= command line parameter may be an acceptable compromise. The alternative is to apply the new default only if AEABI && !OABI_COMPAT. But traditionally, most people building with OABI_COMPAT=y are running EABI userspace anyway. As you say, people running on v5 shouldn't be affected.
So,
Cheers
---Dave
Quoted 53 lines
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This is very much the hard-line approach to the issue, but userspace really shouldn't be depending on the ability to execute architecturally invalid code; ultimately this is an ABI that deserves to be broken. The only trouble with going in softly and just turning on the warning alone is that it should already be rare enough to be easily missed. "Break everything AND scream about it" as the default isn't going to make any friends, but does make bad code as visible as possible.
Anyone is of course free to turn warnings off, and/or fixups back on - defaulting to
UM_SIGNALonUM_IGNOREis to ensure fixup has to be explicitly requested, by which point they should be well aware of what they're doing and why.arch/arm/mm/alignment.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)arch/arm/mm/alignment.c +3
-3@@ -106,10 +106,10 @@ static int safe_usermode(int new_usermode, bool warn)* making any progress. */ if (cpu_is_v6_unaligned () && !(new_usermode & (UM_FIXUP |UM_SIGNAL ))) { - new_usermode |=UM_FIXUP ; + new_usermode |=UM_SIGNAL ;if (warn) -printk (KERN_WARNING "alignment: ignoring faults is unsafe on this CPU. Defaulting to fixup mode.\n" ); +printk (KERN_WARNING "alignment: ignoring faults is unsafe on this CPU. Defaulting to signal mode.\n" ); }return new_usermode;@@ -971,7 +971,7 @@ static int __init alignment_init(void) cr_alignment &= ~CR_A ; cr_no_alignment &= ~CR_A ;set_cr (cr_alignment); - ai_usermode = safe_usermode(ai_usermode, false); + ai_usermode = safe_usermode(ai_usermode | UM_WARN, false); }#endif Signature · 8 lines
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