Re: [PATCH 3/5] perf symbol: Grow the Rust demangle buffer geometrically

[Ian Rogers]

On Sun, Sep 27, 2026 at 6:02 PM Michal Pluta <michalpl2003@xxxxxxxxx> wrote:
>
> When a demangled Rust name doesn't fit in the provided buffer,
> dso__demangle_sym() adds 32 bytes to it and formats the name again from
> the start.  The number of attempts grows with the length of the output,
> so the total work is quadratic.  Names with deeply nested generic types
> need many attempts, leading to noticeable slowdowns in larger programs.
>
> Double the buffer instead, reaching the maximum buffer limit exactly
> rather than stopping 32 bytes early.  The demangled names are
> unchanged.
>
> Add a test for a symbol whose expansion exceeds the bound.
>
> Signed-off-by: Michal Pluta <michalpl2003@xxxxxxxxx>
> ---
>  tools/perf/tests/demangle-rust-v0-test.c | 12 ++++++++++++
>  tools/perf/util/symbol.c                 | 10 ++++++++--
>  2 files changed, 20 insertions(+), 2 deletions(-)
>
> diff --git a/tools/perf/tests/demangle-rust-v0-test.c b/tools/perf/tests/demangle-rust-v0-test.c
> index ee4ddb61174b..d1e0636d73bc 100644
> --- a/tools/perf/tests/demangle-rust-v0-test.c
> +++ b/tools/perf/tests/demangle-rust-v0-test.c
> @@ -69,6 +69,18 @@ static int test__demangle_rust(struct test_suite *test __maybe_unused, int subte
>                 free(buf);
>         }
>
> +       /*
> +        * A symbol with more lifetimes bound than fit in the largest buffer
> +        * must fail to demangle rather than give a truncated name.
> +        */
> +       buf = dso__demangle_sym(/*dso=*/NULL, /*kmodule=*/0, "_RINvC1a1fFGZZZZZZ_EuE");
> +       if (buf) {
> +               pr_debug("FAILED: symbol larger than the buffer limit demangled to %zu bytes\n",
> +                        strlen(buf));
> +               ret = TEST_FAIL;
> +               free(buf);
> +       }
> +
>         return ret;
>  }
>
> diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
> index 3cb42805a82f..1a52bc2980a0 100644
> --- a/tools/perf/util/symbol.c
> +++ b/tools/perf/util/symbol.c
> @@ -2737,29 +2737,35 @@ char *cxx_demangle_sym(const char *str __maybe_unused, bool params __maybe_unuse
>  }
>  #endif /* !HAVE_CXA_DEMANGLE_SUPPORT */
>
> +/* Buffer limit for a demangled Rust symbol name. */
> +#define RUST_DEMANGLE_MAX_LEN (1024 * 1024)
> +
>  char *dso__demangle_sym(struct dso *dso, int kmodule, const char *elf_name)
>  {
>         struct demangle rust_demangle = {
>                 .style = DemangleStyleUnknown,
>         };
>         char *demangled = NULL;
> +       size_t buf_len;

It seems changing the scope of this variable is unnecessary.

>
>         /*
>          * We need to figure out if the object was created from C++ sources
>          * DWARF DW_compile_unit has this, but we don't always have access
>          * to it...
>          */
>         if (!want_demangle((dso && dso__kernel(dso)) || kmodule))
>                 return demangled;
>
>         rust_demangle_demangle(elf_name, &rust_demangle);
>         if (rust_demangle_is_known(&rust_demangle)) {
>                 /* A rust mangled name. */
>                 if (rust_demangle.mangled_len == 0)
>                         return demangled;
>
> -               for (size_t buf_len = roundup_pow_of_two(rust_demangle.mangled_len * 2);
> -                    buf_len < 1024 * 1024; buf_len += 32) {
> +               for (buf_len = min_t(size_t, roundup_pow_of_two(rust_demangle.mangled_len * 2),
> +                                    RUST_DEMANGLE_MAX_LEN);
> +                    buf_len <= RUST_DEMANGLE_MAX_LEN;
> +                    buf_len *= 2) {

Thanks for digging into this problem and exploring a fix! Previously,
we guessed the demangled length was twice the mangled length, then
added 32 bytes for each retry. These were numbers I pulled out of thin
air, so I'm glad you've found them to be wrong :-). Could we estimate
the initial demangled size better? Could you get data from Bevy and
Typst? I'm a little concerned that a demangled symbol of say just over
2KB might require 4KB with this change, instead of 2KB + 32bytes.

Thanks!
Ian

>                         char *tmp = realloc(demangled, buf_len);
>
>                         if (!tmp)
> --
> 2.43.0
>