netdev - Re: [PATCH bpf-next v2 12/15] bpf: Preserve boundaries and track scalars on narrowing fill

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Message-ID: <CAEf4BzaNUfO-EBiJPtOKpBFay2gqr=nzT0Aipe274MG+m2Jvrw@mail.gmail.com>
Date: Tue, 9 Jan 2024 15:51:26 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Maxim Mikityanskiy <maxtram95@...il.com>
Cc: Eduard Zingerman <eddyz87@...il.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	Shung-Hsi Yu <shung-hsi.yu@...e.com>, John Fastabend <john.fastabend@...il.com>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, 
	Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>, 
	"David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Jesper Dangaard Brouer <hawk@...nel.org>, bpf@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	netdev@...r.kernel.org, Maxim Mikityanskiy <maxim@...valent.com>
Subject: Re: [PATCH bpf-next v2 12/15] bpf: Preserve boundaries and track
 scalars on narrowing fill

On Mon, Jan 8, 2024 at 12:53 PM Maxim Mikityanskiy <maxtram95@...il.com> wrote:
>
> From: Maxim Mikityanskiy <maxim@...valent.com>
>
> When the width of a fill is smaller than the width of the preceding
> spill, the information about scalar boundaries can still be preserved,
> as long as it's coerced to the right width (done by coerce_reg_to_size).
> Even further, if the actual value fits into the fill width, the ID can
> be preserved as well for further tracking of equal scalars.
>
> Implement the above improvements, which makes narrowing fills behave the
> same as narrowing spills and MOVs between registers.
>
> Two tests are adjusted to accommodate for endianness differences and to
> take into account that it's now allowed to do a narrowing fill from the
> least significant bits.
>
> reg_bounds_sync is added to coerce_reg_to_size to correctly adjust
> umin/umax boundaries after the var_off truncation, for example, a 64-bit
> value 0xXXXXXXXX00000000, when read as a 32-bit, gets umin = 0, umax =
> 0xFFFFFFFF, var_off = (0x0; 0xffffffff00000000), which needs to be
> synced down to umax = 0, otherwise reg_bounds_sanity_check doesn't pass.
>
> Signed-off-by: Maxim Mikityanskiy <maxim@...valent.com>
> ---
>  include/linux/bpf_verifier.h                  |  2 --
>  include/linux/filter.h                        | 12 ++++++++
>  kernel/bpf/verifier.c                         | 15 +++++++---
>  .../selftests/bpf/progs/verifier_spill_fill.c | 28 +++++++++++++------
>  4 files changed, 42 insertions(+), 15 deletions(-)
>
> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> index e11baecbde68..95ea7657f07e 100644
> --- a/include/linux/bpf_verifier.h
> +++ b/include/linux/bpf_verifier.h
> @@ -239,8 +239,6 @@ enum bpf_stack_slot_type {
>         STACK_ITER,
>  };
>
> -#define BPF_REG_SIZE 8 /* size of eBPF register in bytes */
> -
>  #define BPF_REGMASK_ARGS ((1 << BPF_REG_1) | (1 << BPF_REG_2) | \
>                           (1 << BPF_REG_3) | (1 << BPF_REG_4) | \
>                           (1 << BPF_REG_5))
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 68fb6c8142fe..be784be7ed4e 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -39,6 +39,8 @@ struct sock_reuseport;
>  struct ctl_table;
>  struct ctl_table_header;
>
> +#define BPF_REG_SIZE 8 /* size of eBPF register in bytes */
> +
>  /* ArgX, context and stack frame pointer register positions. Note,
>   * Arg1, Arg2, Arg3, etc are used as argument mappings of function
>   * calls in BPF_CALL instruction.
> @@ -881,6 +883,16 @@ bpf_ctx_narrow_access_offset(u32 off, u32 size, u32 size_default)
>
>  #define bpf_classic_proglen(fprog) (fprog->len * sizeof(fprog->filter[0]))
>
> +static inline bool
> +bpf_stack_narrow_access_ok(int off, int size, int spill_size)

this is used by verifier.c, right? So why not add this to bpf_verifier.h?


nit: given we have spill_size, should we s/size/fill_size/ for symmetry?

> +{
> +#ifdef __BIG_ENDIAN
> +       off -= spill_size - size;
> +#endif
> +
> +       return !(off % BPF_REG_SIZE);
> +}
> +
>  static inline void bpf_prog_lock_ro(struct bpf_prog *fp)
>  {
>  #ifndef CONFIG_BPF_JIT_ALWAYS_ON

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