Re: [PATCH 1/4] perf symbols: Fix broken ELF_C_READ_MMAP fallback guard

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] perf: build failure on systems lacking ELF_C_READ_MMAP enum
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commit d38dd3de1c465f53af2cb4e7df04744c926b7ed6
Author: Alireza Haghdoost <haghdoost@uber.com>

perf symbols: Fix broken ELF_C_READ_MMAP fallback guard

This patch attempts to make perf mmap ELF files by unconditionally defining
PERF_ELF_C_READ_MMAP to ELF_C_READ_MMAP. It removes the previous guard
because ELF_C_READ_MMAP is an enum rather than a preprocessor macro.

> diff --git a/tools/perf/util/symbol.h b/tools/perf/util/symbol.h
> --- a/tools/perf/util/symbol.h
> +++ b/tools/perf/util/symbol.h
> @@ -57,15 +57,8 @@ static inline bool is_livepatch_symbol(const char *str)
>  	return strstarts(str, KLP_SYM_PREFIX);
>  }
>  
> -/*
> - * libelf 0.8.x and earlier do not support ELF_C_READ_MMAP;
> - * for newer versions we can use mmap to reduce memory usage:
> - */
> -#ifdef ELF_C_READ_MMAP
> -# define PERF_ELF_C_READ_MMAP ELF_C_READ_MMAP
> -#else
> -# define PERF_ELF_C_READ_MMAP ELF_C_READ
> -#endif
> +/* libelf falls back to internal reads when mmap fails (e.g. non-seekable fd). */
> +#define PERF_ELF_C_READ_MMAP ELF_C_READ_MMAP

[Severity: Low]
Will this cause compilation errors on systems where the libelf.h header lacks
the ELF_C_READ_MMAP enum?

By unconditionally assigning PERF_ELF_C_READ_MMAP to ELF_C_READ_MMAP in
tools/perf/util/symbol.h, this bypasses build feature detection. On systems
with libelf implementations that do not provide ELF_C_READ_MMAP, such as
elftoolchain on FreeBSD and macOS, this triggers an undeclared identifier
compilation error.

The commit message notes that objtool uses an #ifndef guard which works fine,
but since ELF_C_READ_MMAP is an enum in libelf, that #ifndef in objtool
silently evaluates to true and incorrectly downgrades all mmap reads to
standard reads instead of actually using mmap.

Could we restore a proper feature detection mechanism here rather than
relying on an unconditional definition?

>  
>  #ifdef HAVE_LIBELF_SUPPORT
>  Elf_Scn *elf_section_by_name(Elf *elf, GElf_Ehdr *ep,

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Sashiko AI review · https://sashiko.dev/#/patchset/20260915-perf-symbol-memory-send-v1-0-1d3360e21f07@uber.com?part=1