Re: [PATCH v4 17/39] unwind_user/sframe: Add support for reading .sframe headers

Andrii Nakryiko
2025-02-05T23:05:29Z

On Wed, Feb 5, 2025 at 3:02 AM Jens Remus <jremus@linux.ibm.com> wrote:
>
> On 28.01.2025 02:10, Andrii Nakryiko wrote:
>
> > So, currently in v2, FREs within FDEs use an array-of-structs layout.
> > If we use preudo-C type definitions, it would be something like this
> > for FDE + its FREs:
> >
> > struct FDE_and_FREs {
> > struct sframe_func_desc_entry fde_metadata;
> >
> > union FRE {
> > struct FRE8 {
> > u8 sfre_start_address;
> > u8 sfre_info;
> > u8|u16|u32 offsets[M];
> > }
> > struct FRE16 {
> > u16 sfre_start_address;
> > u16 sfre_info;
> > u8|u16|u32 offsets[M];
> > }
> > struct FRE32 {
> > u32 sfre_start_address;
> > u32 sfre_info;
> > u8|u16|u32 offsets[M];
> > }
> > } fres[N] __packed;
> > };
> >
> > where all fres[i]s are one of those FRE8/FRE16/FRE32, so start
> > addresses have the same size, but each FRE has potentially different
> > offsets sizing, so there is no common alignment, and so everything has
> > to be packed and unaligned.
>
> Just for clarification of the SFrame V2 format, as there may be some
> misconception. Using pseudo-C type definition:
>
> struct sframe_fre8 {
> u8 sfre_start_address;
> u8 sfre_info;
> s8|s16|s32 offsets[M];
> };
> struct sframe_fre16 {
> u16 sfre_start_address;
> u8 sfre_info;
> s8|s16|s32 offsets[M];
> };
> struct sframe_fre32 {
> u32 sfre_start_address;
> u8 sfre_info;
> s8|s16|s32 offsets[M];
> };
>

yeah, sorry, copy/paste error for those sfre_info, it's always u8

> struct sframe_section {
> /* Headers. */
> struct sframe_preamble preamble;
> struct sframe_header header;
> struct sframe_auxiliary_header auxhdr;
>
> /* FDEs. */
> struct sframe_fde fdes[N_FDE];
>
> /* FRE8s / FRE16s / FRE32s per FDE. */
> struct sframe_fre{8|16|32} fres_fde1[N_FRE_FDE1] __packed;
> ...
> struct sframe_fre{8|16|32} fres_fdeN[N_FRE_FDEN] __packed;
> };
>
> Where:
> - fdes[] can be binary searched: All fdes[i] are of equal size and
> sorted on start address.
> - Each fdes[i] points at its fres_fdei[].
> - fres_fdei[] cannot be binary searched: For each fdes[i] they are
> one of those FRE8/FRE16/FRE32, so start addresses have the same
> size, but each FRE has potentially different offsets sizing, so
> there is no common alignment, and so everything has to be packed
> and unaligned.
>

yep

> Regards,
> Jens
> --
> Jens Remus
> Linux on Z Development (D3303)
> +49-7031-16-1128 Office
> jremus@de.ibm.com
>
> IBM
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