[RFC PATCH] ARM: Use generic BUG() handler
Dave Martin
dave.martin at linaro.org
Thu Mar 3 09:44:15 EST 2011
On Thu, Mar 3, 2011 at 2:11 PM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> On Wed, Mar 02, 2011 at 10:59:50AM +0000, Dave Martin wrote:
>> + asm volatile("1:\t" __bug_instr "\n" q \
>> + ".pushsection .rodata.str, \"a\"\n" \
>> + "2:\t.asciz \"" #__file "\"\n" \
>> + ".popsection\n" \
>
> Doesn't this mean we end up with multiple file names?
Hmmm, yes, you're right.
After a bit of digging in the documentation, I find that ELF supports
mergable string sections though, so
.pushsection .rodata.str, "aMS", 1
.asciz __FILE__
.popsection
...actually looks like it ought to do the right thing: duplicate
strings in the section get merged during linking, even if there are
duplicates from a single object.
Simple experiments suggest that the linker does this right, even
merging distinct strings which share a common suffix. Do we use this
elsewhere in the kernel? It it's not already used, it could be a win
for any large, static string tables.
>
>> + ".pushsection __bug_table,\"a\"\n" \
>> + "3:\t.word 1b, 2b\n" \
>> + "\t.hword " #__line ", 0\n" \
>> + ".popsection" \
>> + unreachable(); \
>> +} while (0)
>
> Second problem is that the above produces this:
>
> 1: .word 0xec000000
> .pushsection .rodata.str, "a"
> 2: .asciz "__FILE__"
> .popsection
> .pushsection __bug_table,"a"
> 3: .word 1b, 2b
> .hword __LINE__, 0
> .popsection
>
> which is clearly not what we want.
Indeed. I did say I hadn't tested it :)
> Adding another level of indirection
> starts to get closer to what we desire:
>
> 1: .word 0xec000000
> .pushsection .rodata.str, "a"
> 2: .asciz ""t.c""
> .popsection
> .pushsection __bug_table,"a"
> 3: .word 1b, 2b
> .hword 19, 0
> .popsection
>
> but we're still ending up with multiple strings containing the filename,
> which is going to excessively bloat the kernel no end.
>
We'd need to remove the duplicate quotes, too.
For the .org <blah> + sizeof (struct bug_entry) problem, Will pointed
out that there's a BUILD_BUG_ON() macro which we could used to avoid
silently producing a bad build if the result of that sizeof isn't what
we expect.
So I still think it could work ... but it's a bit of an ugly hack I confess.
Cheers
---Dave
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