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bpf: support resilient split BTF #7016

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bpf: support resilient split BTF #7016

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Pull request for series with
subject: bpf: support resilient split BTF
version: 3
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=852223

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Upstream branch: fcd1ed8
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=852223
version: 3

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Upstream branch: fcd1ed8
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=852223
version: 3

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Upstream branch: fcd1ed8
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=852223
version: 3

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Upstream branch: 80c5a07
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=852223
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Upstream branch: 20a759d
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Upstream branch: bbe91a9
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=852223
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Upstream branch: 792a04b
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Upstream branch: a3c1c95
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Upstream branch: 6a2f786
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Upstream branch: ba39486
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Upstream branch: 83eea61
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Upstream branch: 71ed6c2
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Upstream branch: 71ed6c2
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=853939
version: 4

@alan-maguire
libbpf: add btf__distill_base() creating split BTF with distilled bas…
cc71ec9 
…e BTF

To support more robust split BTF, adding supplemental context for the
base BTF type ids that split BTF refers to is required.  Without such
references, a simple shuffling of base BTF type ids (without any other
significant change) invalidates the split BTF.  Here the attempt is made
to store additional context to make split BTF more robust.

This context comes in the form of distilled base BTF providing minimal
information (name and - in some cases - size) for base INTs, FLOATs,
STRUCTs, UNIONs, ENUMs and ENUM64s along with modified split BTF that
points at that base and contains any additional types needed (such as
TYPEDEF, PTR and anonymous STRUCT/UNION declarations).  This
information constitutes the minimal BTF representation needed to
disambiguate or remove split BTF references to base BTF.  The rules
are as follows:

- INT, FLOAT are recorded in full.
- if a named base BTF STRUCT or UNION is referred to from split BTF, it
  will be encoded either as a zero-member sized STRUCT/UNION (preserving
  size for later relocation checks) or as a named FWD.  Only base BTF
  STRUCT/UNIONs that are either embedded in split BTF STRUCT/UNIONs or
  that have multiple STRUCT/UNION instances of the same name need to
  preserve size information, so a FWD representation will be used in
  most cases.
- if an ENUM[64] is named, a ENUM forward representation (an ENUM
  with no values) is used.
- in all other cases, the type is added to the new split BTF.

Avoiding struct/union/enum/enum64 expansion is important to keep the
distilled base BTF representation to a minimum size.

When successful, new representations of the distilled base BTF and new
split BTF that refers to it are returned.  Both need to be freed by the
caller.

So to take a simple example, with split BTF with a type referring
to "struct sk_buff", we will generate distilled base BTF with a
FWD struct sk_buff, and the split BTF will refer to it instead.

Tools like pahole can utilize such split BTF to populate the .BTF
section (split BTF) and an additional .BTF.base section.  Then
when the split BTF is loaded, the distilled base BTF can be used
to relocate split BTF to reference the current (and possibly changed)
base BTF.

So for example if "struct sk_buff" was id 502 when the split BTF was
originally generated,  we can use the distilled base BTF to see that
id 502 refers to a "struct sk_buff" and replace instances of id 502
with the current (relocated) base BTF sk_buff type id.

Distilled base BTF is small; when building a kernel with all modules
using distilled base BTF as a test, ovreall module size grew by only
5.3Mb total across ~2700 modules.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
@alan-maguire
selftests/bpf: test distilled base, split BTF generation
e3b1726 
Test generation of split+distilled base BTF, ensuring that

- base BTF STRUCTs which are embedded in split BTF structs are
  represented as 0-member sized structs, allowing size checking
- base BTF UNION which has a duplicate-named UNION is represented
  as a 0-member sized union, helping clarify which union is referred
  to
- FWDs are used in place of full named struct/union declarations
- FWDs are used in place of full named enum declarations
- anonymous struct/unions are represented in full in split BTF
- anonymous enums are represented in full in split BTF
- types unreferenced from split BTF are not present in distilled
  base BTF

Also test that with vmlinux BTF and split BTF based upon it,
we only represent needed base types referenced from split BTF
in distilled base.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
@alan-maguire
libbpf: add btf__parse_opts() API for flexible BTF parsing
0d2aa5a 
Options cover existing parsing scenarios (ELF, raw, retrieving
.BTF.ext) and also allow specification of the ELF section name
containing BTF.  This will allow consumers to retrieve BTF from
.BTF.base sections (BTF_BASE_ELF_SEC) also.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
@alan-maguire
bpftool: support displaying raw split BTF using base BTF section as base
53bd06c 
If no base BTF can be found, fall back to checking for the .BTF.base
section and use it to display split BTF.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Quentin Monnet <qmo@kernel.org>
@alan-maguire
resolve_btfids: use .BTF.base ELF section as base BTF if -B option is…
e2226b8 
… used

When resolving BTF ids, use the BTF in the module .BTF.base section
when passed the -B option.  Both references to base BTF from split
BTF and BTF ids will be relocated for base vmlinux on module load.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
@alan-maguire
kbuild, bpf: add module-specific pahole/resolve_btfids flags for dist…
65ebb9e 
…illed base BTF

Support creation of module BTF along with distilled base BTF;
the latter is stored in a .BTF.base ELF section and supplements
split BTF references to base BTF with information about base types,
allowing for later relocation of split BTF with a (possibly
changed) base.  resolve_btfids uses the "-B" option to specify
that the BTF.ids section should be populated with split BTF
relative to the added .BTF.base section rather than relative
to the vmlinux base.

Modules will be built with a distilled .BTF.base section for external
module build, i.e.

make -C. -M=path2/module

...while in-tree module build as part of a normal kernel build will
not generate distilled base BTF; this is because in-tree modules
change with the kernel and do not require BTF relocation for the
running vmlinux.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
@alan-maguire
libbpf: split BTF relocation
bd4e8cc 
Map distilled base BTF type ids referenced in split BTF and their
references to the base BTF passed in, and if the mapping succeeds,
reparent the split BTF to the base BTF.

Relocation is done by first verifying that distilled base BTF
only consists of named INT, FLOAT, ENUM, FWD, STRUCT and
UNION kinds; then we sort these to speed lookups.  Once sorted,
the base BTF is iterated, and for each relevant kind we check
for an equivalent in distilled base BTF.  When found, the
mapping from distilled -> base BTF id and string offset is recorded.

Once all mappings are established, we can update type ids
and string offsets in split BTF and reparent it to the new base.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
@alan-maguire
selftests/bpf: extend distilled BTF tests to cover BTF relocation
1c55ef8 
Ensure relocated BTF looks as expected; in this case identical to
original split BTF, with a few duplicate anonymous types added to
split BTF by the relocation process.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
@alan-maguire
module, bpf: store BTF base pointer in struct module
72f4be8 
...as this will allow split BTF modules with a base BTF
representation (rather than the full vmlinux BTF at time of
BTF encoding) to resolve their references to kernel types in a
way that is more resilient to small changes in kernel types.

This will allow modules that are not built every time the kernel
is to provide more resilient BTF, rather than have it invalidated
every time BTF ids for core kernel types change.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
@alan-maguire
libbpf,bpf: share BTF relocate-related code with kernel
e2a19a1 
Share relocation implementation with the kernel.  As part of this,
we also need the type/string visitation functions so add them to a
btf_common.c file that also gets shared with the kernel. Relocation
code in kernel and userspace is identical save for the impementation
of the reparenting of split BTF to the relocated base BTF and
retrieval of BTF header from "struct btf"; these small functions
need separate user-space and kernel implementations.

One other wrinkle on the kernel side is we have to map .BTF.ids in
modules as they were generated with the type ids used at BTF encoding
time. btf_relocate() optionally returns an array mapping from old BTF
ids to relocated ids, so we use that to fix up these references where
needed for kfuncs.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
@alan-maguire
bpftool: support displaying relocated-with-base split BTF
98275e0 
If the -R <base_btf> option is used, we can display BTF that has been
generated with distilled base BTF in its relocated form.  For example
for bpf_testmod.ko (which is built as an out-of-tree module, so has
a distilled .BTF.base section:

bpftool btf dump file bpf_testmod.ko

Alternatively, we can display content relocated with
(a possibly changed) base BTF via

bpftool btf dump -R /sys/kernel/btf/vmlinux bpf_testmod.ko

The latter mirrors how the kernel will handle such split
BTF; it relocates its representation with the running
kernel, and if successful, renumbers BTF ids to reference
the current vmlinux BTF.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Reviewed-by: Quentin Monnet <qmo@kernel.org>
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Upstream branch: 6f130e4
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=853939
version: 4

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At least one diff in series https://patchwork.kernel.org/project/netdevbpf/list/?series=853939 expired. Closing PR.

@kernel-patches-daemon-bpf kernel-patches-daemon-bpf bot deleted the series/837288=>bpf-next branch May 22, 2024 22:20
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