GCC jump tables

GCC jump tables

[Andrey Grodzovsky]

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Hello, question regarding jump tables - is there a way to totally 
disable their generation in C code ? I know -fno-jump-tables but it's 
only effective for switch cases, but still after  compiling my code with 
this flag i see a jump table being generated due to some function call. 
Is there any way to totally prevent their creation ?

Thanks,
Andrey


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Re: GCC jump tables

[Segher Boessenkool]

Hi!

On Mon, Aug 07, 2023 at 10:59:51PM -0400, Andrey Grodzovsky via Gcc-help wrote:
> Hello, question regarding jump tables - is there a way to totally 
> disable their generation in C code ? I know -fno-jump-tables but it's 
> only effective for switch cases, but still after  compiling my code with 
> this flag i see a jump table being generated due to some function call. 
> Is there any way to totally prevent their creation ?

For what other source code do you see jump tables generated?  Or, let's
take a step back first: what exactly do you call a "jump table" here?


Segher

Re: [External] Re: GCC jump tables

[Andrey Grodzovsky]

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On 2023-08-08 10:54, Segher Boessenkool wrote:
> Hi!
>
> On Mon, Aug 07, 2023 at 10:59:51PM -0400, Andrey Grodzovsky via Gcc-help wrote:
>> Hello, question regarding jump tables - is there a way to totally
>> disable their generation in C code ? I know -fno-jump-tables but it's
>> only effective for switch cases, but still after  compiling my code with
>> this flag i see a jump table being generated due to some function call.
>> Is there any way to totally prevent their creation ?
> For what other source code do you see jump tables generated?


It's calling any of the get_user_pages_remote, get_user_pages, put_page, 
mmap_read_lock, mmap_read_unlock function calls,
I am calling them from within kernel module, they reside in the 
monolitic vmlinux kernel binary and are exported outside
for kernel modules use. e.g. Definition of  get_user_pages is here 
https://elixir.bootlin.com/linux/v5.15/source/mm/gup.c#L1964


> Or, let's
> take a step back first: what exactly do you call a "jump table" here?


For me it would be anything that ends up creating a jump table entry in 
the ELF header or the resulting object file so this when
i run readelf -S on the .o file

   [20] __jump_table      PROGBITS         0000000000000000 00001d00
   [21] .rela__jump_table RELA             0000000000000000 0002aaf0


If i comment out those calls then the entries above disappear.

Thanks,
Andrey


>
>
> Segher



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Re: [External] Re: GCC jump tables

[Segher Boessenkool]

On Tue, Aug 08, 2023 at 11:14:02AM -0400, Andrey Grodzovsky wrote:
> On 2023-08-08 10:54, Segher Boessenkool wrote:
> >On Mon, Aug 07, 2023 at 10:59:51PM -0400, Andrey Grodzovsky via Gcc-help 
> >wrote:
> >>Hello, question regarding jump tables - is there a way to totally
> >>disable their generation in C code ? I know -fno-jump-tables but it's
> >>only effective for switch cases, but still after  compiling my code with
> >>this flag i see a jump table being generated due to some function call.
> >>Is there any way to totally prevent their creation ?
> >For what other source code do you see jump tables generated?
> 
> It's calling any of the get_user_pages_remote, get_user_pages, put_page, 
> mmap_read_lock, mmap_read_unlock function calls,
> I am calling them from within kernel module, they reside in the 
> monolitic vmlinux kernel binary and are exported outside
> for kernel modules use. e.g. Definition of  get_user_pages is here 
> https://elixir.bootlin.com/linux/v5.15/source/mm/gup.c#L1964
> 
> >Or, let's
> >take a step back first: what exactly do you call a "jump table" here?
> 
> For me it would be anything that ends up creating a jump table entry in 
> the ELF header or the resulting object file so this when

This is stuff that the Linux kernel does.  Manually.  The compiler has
nothing to do with it.


Segher

Re: [External] Re: [External] Re: GCC jump tables

[Andrey Grodzovsky]

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On 2023-08-08 16:48, Segher Boessenkool wrote:
> On Tue, Aug 08, 2023 at 11:14:02AM -0400, Andrey Grodzovsky wrote:
>> On 2023-08-08 10:54, Segher Boessenkool wrote:
>>> On Mon, Aug 07, 2023 at 10:59:51PM -0400, Andrey Grodzovsky via Gcc-help
>>> wrote:
>>>> Hello, question regarding jump tables - is there a way to totally
>>>> disable their generation in C code ? I know -fno-jump-tables but it's
>>>> only effective for switch cases, but still after  compiling my code with
>>>> this flag i see a jump table being generated due to some function call.
>>>> Is there any way to totally prevent their creation ?
>>> For what other source code do you see jump tables generated?
>> It's calling any of the get_user_pages_remote, get_user_pages, put_page,
>> mmap_read_lock, mmap_read_unlock function calls,
>> I am calling them from within kernel module, they reside in the
>> monolitic vmlinux kernel binary and are exported outside
>> for kernel modules use. e.g. Definition of  get_user_pages is here
>> https://urldefense.proofpoint.com/v2/url?u=https-3A__elixir.bootlin.com_linux_v5.15_source_mm_gup.c-23L1964&d=DwIDAw&c=aC52l5ychvsxSLm72twIgEW2BEeqYPY0uTQnrVZ6RBM&r=MPhNfvPJjusxPWFiIwhytlSBR8Vj8O1RcqwrEfh5BQ7jv0n8sh4iBBnJso-aIHAA&m=-_eY5Xylio3iSKZ0qoybyl-kdPuN3EDl1o7eQhcMEzkzScMNML1FGQrPEelgvGhT&s=prTAoPR06auK9EH2ixHIy5SdiVRW5y3jXXsjhaFdKgs&e=
>>
>>> Or, let's
>>> take a step back first: what exactly do you call a "jump table" here?
>> For me it would be anything that ends up creating a jump table entry in
>> the ELF header or the resulting object file so this when
> This is stuff that the Linux kernel does.  Manually.  The compiler has
> nothing to do with it.


Yep, already found the needed flag to #undef to disable jump tables 
emission - https://elixir.bootlin.com/linux/v5.15/source/arch/Kconfig#L48

Andrey


>
>
> Segher



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