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SubjectRe: [PATCH v3 04/13] recordmcount: Rewrite error/success handling
On Wed, 24 Jul 2019 14:04:58 -0700
Matt Helsley <mhelsley@vmware.com> wrote:


Hi Matt,

As I'm applying these for real, I'm taking a deeper look at the
patches. This one I have some questions about.

> Recordmcount uses setjmp/longjmp to manage control flow as
> it reads and then writes the ELF file. This unusual control
> flow is hard to follow and check in addition to being unlike
> kernel coding style.
>
> So we rewrite these paths to use regular return values to
> indicate error/success. When an error or previously-completed object
> file is found we return an error code following kernel
> coding conventions -- negative error values and 0 for success when
> we're not returning a pointer. We return NULL for those that fail
> and return non-NULL pointers otherwise.
>
> One oddity is already_has_rel_mcount -- there we use pointer comparison
> rather than string comparison to differentiate between
> previously-processed object files and returning the name of a text
> section.

This is fine, but it's got a strange implementation.



> diff --git a/scripts/recordmcount.h b/scripts/recordmcount.h
> index c1e1b04b4871..909a3e4775c2 100644
> --- a/scripts/recordmcount.h
> +++ b/scripts/recordmcount.h
> @@ -24,7 +24,9 @@
> #undef mcount_adjust
> #undef sift_rel_mcount
> #undef nop_mcount
> +#undef missing_sym
> #undef find_secsym_ndx
> +#undef already_has_rel_mcount

Why do we need these as defines? Can't you just have a single:

const char *already_has_mcount = "success";

in recordmcount.c before recordmcount.h is included?

And same for missing_sym.

Another, probably more robust way of doing this, is change
find_secsym_ndx() to return 0 on success and -1 on missing symbol, and
just pass a pointer by reference to fill the recsym (which doesn't have
to be a constant).

I've applied the first 3 patches of this series, but stopped here. I'll
continue to take a deeper look at your other patches.

Thanks!

-- Steve



> #undef __has_rel_mcount
> #undef has_rel_mcount
> #undef tot_relsize
> @@ -54,7 +56,9 @@
> # define append_func append64
> # define sift_rel_mcount sift64_rel_mcount
> # define nop_mcount nop_mcount_64
> +# define missing_sym missing_sym_64
> # define find_secsym_ndx find64_secsym_ndx
> +# define already_has_rel_mcount already_has_rel_mcount_64
> # define __has_rel_mcount __has64_rel_mcount
> # define has_rel_mcount has64_rel_mcount
> # define tot_relsize tot64_relsize
> @@ -87,7 +91,9 @@
> # define append_func append32
> # define sift_rel_mcount sift32_rel_mcount
> # define nop_mcount nop_mcount_32
> +# define missing_sym missing_sym_32
> # define find_secsym_ndx find32_secsym_ndx
> +# define already_has_rel_mcount already_has_rel_mcount_32
> # define __has_rel_mcount __has32_rel_mcount
> # define has_rel_mcount has32_rel_mcount
> # define tot_relsize tot32_relsize

> +static const unsigned int missing_sym = (unsigned int)-1;
>
> /*
> * Find a symbol in the given section, to be used as the base for relocating
> @@ -443,9 +469,11 @@ static unsigned find_secsym_ndx(unsigned const txtndx,
> }
> fprintf(stderr, "Cannot find symbol for section %u: %s.\n",
> txtndx, txtname);
> - fail_file();
> + cleanup();
> + return missing_sym;
> }
>
> +char const *already_has_rel_mcount = "success"; /* our work here is done! */
>
> /* Evade ISO C restriction: no declaration after statement in has_rel_mcount. */
> static char const *
> @@ -461,7 +489,8 @@ __has_rel_mcount(Elf_Shdr const *const relhdr, /* is SHT_REL or SHT_RELA */
> if (strcmp("__mcount_loc", txtname) == 0) {
> fprintf(stderr, "warning: __mcount_loc already exists: %s\n",
> fname);
> - succeed_file();
> + cleanup();
> + return already_has_rel_mcount;
> }
> if (w(txthdr->sh_type) != SHT_PROGBITS ||
> !(_w(txthdr->sh_flags) & SHF_EXECINSTR))

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