Re: [PATCH] perf trace: Refactor augmented_raw_syscalls using bpf_loop

[Andrii Nakryiko]

On Wed, Jun 24, 2026 at 3:27 AM Viktor Malik <vmalik@xxxxxxxxxx> wrote:
>
> On 6/24/26 08:47, Viktor Malik wrote:
> > On 6/23/26 19:10, Namhyung Kim wrote:
> >> Hello,
> >>
> >> On Tue, Jun 23, 2026 at 08:27:39AM -0700, Alexei Starovoitov wrote:
> >>> On Tue Jun 23, 2026 at 4:25 AM PDT, Viktor Malik wrote:
> >>>> The loop for processing syscall args in augment_raw_syscalls has a
> >>>> history of breaking with Clang updates, see e.g. commit 013eb043f37b
> >>>> ("perf trace: Fix BPF loading failure (-E2BIG)") from Clang 15 to 16.
> >>>>
> >>>> Now, a similar thing happened between Clang 21 and 22. While the issue
> >>>> is mitigated on the main line by a recent verifier update, it remains
> >>>> broken on the 6.12 and 6.18 stable branches:
> >>>>
> >>>>     [linux-6.18.y]# sudo perf trace true
> >>>>     libbpf: prog 'sys_enter': BPF program load failed: -E2BIG
> >>>>     libbpf: prog 'sys_enter': -- BEGIN PROG LOAD LOG --
> >>>>     [...]
> >>>>     BPF program is too large. Processed 1000001 insn
> >>>>     processed 1000001 insns (limit 1000000) max_states_per_insn 40 total_states 37941 peak_states 232 mark_read 0
> >>>>     -- END PROG LOAD LOG --
> >>>>     libbpf: prog 'sys_enter': failed to load: -E2BIG
> >>>>     libbpf: failed to load object 'augmented_raw_syscalls_bpf'
> >>>>     libbpf: failed to load BPF skeleton 'augmented_raw_syscalls_bpf': -E2BIG
> >>>>     Error: failed to get syscall or beauty map fd
> >>>>     [...]
> >>>>
> >>>> The reason is that the loop is quite complex and the BPF verifier often
> >>>> struggles to prove that it terminates.
> >>>>
> >>>> Fix the issue by refactoring the loop body into a callback function and
> >>>> calling the bpf_loop helper. This should prevent future breakages of
> >>>> this kind since the callback function has no loops. It also allows to
> >>>> drop a few artificial checks to help the verifier, including the changes
> >>>> introduced by 013eb043f37b.
> >>
> >> Thanks for working on this.  I encountered this issue before and never
> >> found time to take a deeper look yet.
> >>
> >>>>
> >>>> Signed-off-by: Viktor Malik <vmalik@xxxxxxxxxx>
> >>>> Fixes: a68fd6a6cdd3 ("perf trace: Collect augmented data using BPF")
> >>>> Fixes: 013eb043f37b ("perf trace: Fix BPF loading failure (-E2BIG)")
> >>>> Cc: stable@xxxxxxxxxxxxxxx
> >>>> ---
> >>>>  .../bpf_skel/augmented_raw_syscalls.bpf.c     | 157 +++++++++++-------
> >>>>  1 file changed, 96 insertions(+), 61 deletions(-)
> >>>>
> >>>> diff --git a/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c b/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c
> >>>> index 2a6e61864ee0..6d553ed3ac23 100644
> >>>> --- a/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c
> >>>> +++ b/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c
> >>>> @@ -429,15 +429,96 @@ static bool pid_filter__has(struct pids_filtered *pids, pid_t pid)
> >>>>    return bpf_map_lookup_elem(pids, &pid) != NULL;
> >>>>  }
> >>>>
> >>>> +struct args_loop_ctx {
> >>>> +  struct syscall_enter_args *args;
> >>>> +  unsigned int *beauty_map;
> >>>> +  void *payload_offset;
> >>>> +  int value_size;
> >>>> +  u64 *output;
> >>>> +  bool *do_output;
> >>>> +};
> >>>> +
> >>>> +static long process_arg_cb(u64 i, void *ctx)
> >>>> +{
> >>>> +  /*
> >>>> +   * Determine what type of argument and how many bytes to read from user space, using the
> >>>> +   * value in the beauty_map. This is the relation of parameter type and its corresponding
> >>>> +   * value in the beauty map, and how many bytes we read eventually:
> >>>> +   *
> >>>> +   * string: 1                          -> size of string
> >>>> +   * struct: size of struct             -> size of struct
> >>>> +   * buffer: -1 * (index of paired len) -> value of paired len (maximum: TRACE_AUG_MAX_BUF)
> >>>> +   */
> >>>> +  struct augmented_arg *augmented_arg;
> >>>> +  struct args_loop_ctx *loop_ctx;
> >>>> +  int aug_size, size, index;
> >>>> +  bool augmented;
> >>>> +  void *arg;
> >>>> +
> >>>> +  /* Bounds check for the below map access to help the verifier */
> >>>> +  if (i < 0 || i >= 6)
> >>>> +          return 1;
> >>>> +
> >>>> +  loop_ctx = (struct args_loop_ctx *)ctx;
> >>>> +  arg = (void *)loop_ctx->args->args[i];
> >>>> +  augmented = false;
> >>>> +  size = loop_ctx->beauty_map[i];
> >>>> +  aug_size = size; /* size of the augmented data read from user space */
> >>>> +  augmented_arg = (struct augmented_arg *)loop_ctx->payload_offset;
> >>>> +
> >>>> +  if (size == 0 || arg == NULL)
> >>>> +          return 0; /* continue */
> >>>> +
> >>>> +  if (size == 1) { /* string */
> >>>> +          aug_size = bpf_probe_read_user_str(augmented_arg->value, loop_ctx->value_size, arg);
> >>>> +          augmented = true;
> >>>> +  } else if (size > 0 && size <= loop_ctx->value_size) { /* struct */
> >>>> +          if (!bpf_probe_read_user(augmented_arg->value, size, arg))
> >>>> +                  augmented = true;
> >>>> +  } else if (size < 0 && size >= -6) { /* buffer */
> >>>> +          index = -(size + 1);
> >>>> +          barrier_var(index); // Prevent clang (noticed with v18) from removing the &= 7 trick.
> >>>> +          index &= 7;         // Satisfy the bounds checking with the verifier in some kernels.
> >>>> +          aug_size = loop_ctx->args->args[index];
> >>>> +
> >>>> +          if (aug_size > TRACE_AUG_MAX_BUF)
> >>>> +                  aug_size = TRACE_AUG_MAX_BUF;
> >>>> +
> >>>> +          if (aug_size > 0) {
> >>>> +                  if (!bpf_probe_read_user(augmented_arg->value, aug_size, arg))
> >>>> +                          augmented = true;
> >>>> +          }
> >>>> +  }
> >>>> +
> >>>> +  /* Augmented data size is limited to sizeof(augmented_arg->unnamed union with value field) */
> >>>> +  if (aug_size > loop_ctx->value_size)
> >>>> +          aug_size = loop_ctx->value_size;
> >>>> +
> >>>> +  /* write data to payload */
> >>>> +  if (augmented) {
> >>>> +          int written = offsetof(struct augmented_arg, value) + aug_size;
> >>>> +
> >>>> +          if (written < 0 || written > sizeof(struct augmented_arg))
> >>>> +                  return 1; /* break */
> >>>> +
> >>>> +          augmented_arg->size = aug_size;
> >>>> +          *loop_ctx->output += written;
> >>>> +          loop_ctx->payload_offset += written;
> >>>> +          *loop_ctx->do_output = true;
> >>>> +  }
> >>>> +
> >>>> +  return 0;
> >>>> +}
> >>>> +
> >>>>  static int augment_sys_enter(void *ctx, struct syscall_enter_args *args)
> >>>>  {
> >>>> -  bool augmented, do_output = false;
> >>>> -  int zero = 0, index, value_size = sizeof(struct augmented_arg) - offsetof(struct augmented_arg, value);
> >>>> +  bool do_output = false;
> >>>> +  int zero = 0, value_size = sizeof(struct augmented_arg) - offsetof(struct augmented_arg, value);
> >>>>    u64 output = 0; /* has to be u64, otherwise it won't pass the verifier */
> >>>> -  s64 aug_size, size;
> >>>>    unsigned int nr, *beauty_map;
> >>>>    struct beauty_payload_enter *payload;
> >>>> -  void *arg, *payload_offset;
> >>>> +  void *payload_offset;
> >>>> +  long iters;
> >>>>
> >>>>    /* fall back to do predefined tail call */
> >>>>    if (args == NULL)
> >>>> @@ -457,63 +538,17 @@ static int augment_sys_enter(void *ctx, struct syscall_enter_args *args)
> >>>>    /* copy the sys_enter header, which has the syscall_nr */
> >>>>    __builtin_memcpy(&payload->args, args, sizeof(struct syscall_enter_args));
> >>>>
> >>>> -  /*
> >>>> -   * Determine what type of argument and how many bytes to read from user space, using the
> >>>> -   * value in the beauty_map. This is the relation of parameter type and its corresponding
> >>>> -   * value in the beauty map, and how many bytes we read eventually:
> >>>> -   *
> >>>> -   * string: 1                          -> size of string
> >>>> -   * struct: size of struct             -> size of struct
> >>>> -   * buffer: -1 * (index of paired len) -> value of paired len (maximum: TRACE_AUG_MAX_BUF)
> >>>> -   */
> >>>> -  for (int i = 0; i < 6; i++) {
> >>>> -          arg = (void *)args->args[i];
> >>>> -          augmented = false;
> >>>> -          size = beauty_map[i];
> >>>> -          aug_size = size; /* size of the augmented data read from user space */
> >>>> -
> >>>> -          if (size == 0 || arg == NULL)
> >>>> -                  continue;
> >>>> -
> >>>> -          if (size == 1) { /* string */
> >>>> -                  aug_size = bpf_probe_read_user_str(((struct augmented_arg *)payload_offset)->value, value_size, arg);
> >>>> -                  /* minimum of 0 to pass the verifier */
> >>>> -                  if (aug_size < 0)
> >>>> -                          aug_size = 0;
> >>>> -
> >>>> -                  augmented = true;
> >>>> -          } else if (size > 0 && size <= value_size) { /* struct */
> >>>> -                  if (!bpf_probe_read_user(((struct augmented_arg *)payload_offset)->value, size, arg))
> >>>> -                          augmented = true;
> >>>> -          } else if ((int)size < 0 && size >= -6) { /* buffer */
> >>>> -                  index = -(size + 1);
> >>>> -                  barrier_var(index); // Prevent clang (noticed with v18) from removing the &= 7 trick.
> >>>> -                  index &= 7;         // Satisfy the bounds checking with the verifier in some kernels.
> >>>> -                  aug_size = args->args[index] > TRACE_AUG_MAX_BUF ? TRACE_AUG_MAX_BUF : args->args[index];
> >>>> -
> >>>> -                  if (aug_size > 0) {
> >>>> -                          if (!bpf_probe_read_user(((struct augmented_arg *)payload_offset)->value, aug_size, arg))
> >>>> -                                  augmented = true;
> >>>> -                  }
> >>>> -          }
> >>>> -
> >>>> -          /* Augmented data size is limited to sizeof(augmented_arg->unnamed union with value field) */
> >>>> -          if (aug_size > value_size)
> >>>> -                  aug_size = value_size;
> >>>> -
> >>>> -          /* write data to payload */
> >>>> -          if (augmented) {
> >>>> -                  int written = offsetof(struct augmented_arg, value) + aug_size;
> >>>> -
> >>>> -                  if (written < 0 || written > sizeof(struct augmented_arg))
> >>>> -                          return 1;
> >>>> -
> >>>> -                  ((struct augmented_arg *)payload_offset)->size = aug_size;
> >>>> -                  output += written;
> >>>> -                  payload_offset += written;
> >>>> -                  do_output = true;
> >>>> -          }
> >>>> -  }
> >>>> +  struct args_loop_ctx loop_ctx = {
> >>>> +          .args = args,
> >>>> +          .beauty_map = beauty_map,
> >>>> +          .payload_offset = payload_offset,
> >>>> +          .value_size = value_size,
> >>>> +          .output = &output,
> >>>> +          .do_output = &do_output
> >>>> +  };
> >>>> +  iters = bpf_loop(6, process_arg_cb, &loop_ctx, 0);
> >>>
> >>> bpf_loop() is old and generally not recommended.
> >>> Please use bpf_for() then the diff will be one line change and
> >>> can scale to any number of args. Not just 6.
> >
> > Thanks Alexei, I didn't know about this preference.
> >
> >> One thing we should take care is to support old kernels.  The oldest
> >> LTS kernel in the kernel.org is 5.10 and bpf_loop() was introduced in
> >> 5.17 and bpf_for (bpf_iter_num) was 6.4.
> >
> > The problematic loop was introduced in 6.12 by a68fd6a6cdd3 ("perf
> > trace: Collect augmented data using BPF") so we should be good using
> > bpf_for. Or is perf from 7.2 supposed to work on 5.10 LTS kernels?
> >
> > I'll refactor with bpf_for and will send v2.
>
> Or I won't. It turns out that just swapping the for loop for bpf_for
> leads to -E2BIG from the verifier again. Looking at the verifier log, it
> fails to find equivalence between states at the loop head:
>
>     [...]
>     78: (85) call bpf_iter_num_next#84922 [...]
> fp-56=map_value(map=beauty_payload_,ks=4,vs=24688,imm=112)
>     [...]
>     78: (85) call bpf_iter_num_next#84922 [...]
> fp-56=map_value(map=beauty_payload_,ks=4,vs=24688,imm=120)
>     [...]
>
> IMHO, the reason is that payload_offset, which points to the
> beauty_payload_enter_map entry, gets updated in every iteration.
>
> This could be probably fixed on the perf side by reworking how augmented
> args are stored but at this point, bpf_loop sounds like an easier and
> more reliable approach.
>
> Let me know if anyone has objections, otherwise I'll send v2 of the
> bpf_loop approach, with suggestions from Sashiko incorporated.
>

I'd still try to adapt bpf_for(), it's a much better code structure.
You probably need to add a bounding checking/confirming `if ()`
condition validating that offset at which you access map_value is
always correct. And/or you might need barrier_var() before using i,
because bpf_for() macro does bounds checking (check the macro itself),
but compiler often will reorder instructions leading to verifier
complaints.

> Thanks,
> Viktor
>
> > It should be then
> > backported to stable kernels down to 6.12 LTS.
> >
> > Viktor
> >
> >>
> >> Maybe we can factor out the loop body and call it from different
> >> mechanisms like open-coded loop, bpf_loop or bpf_for depending on the
> >> kernel version.  But not sure it'd fix the verifier issue though.
> >>
> >> Thanks,
> >> Namhyung
> >>
>
>