Re: [PATCH dwarves v3 2/5] btf_encoder: Do not use both structs and pointers for the same data

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Giuliano Procida <gprocida@google.com>
Cc: dwarves@vger.kernel.org,
	"Arnaldo Carvalho de Melo" <acme@kernel.org>,
	"Andrii Nakryiko" <andrii@kernel.org>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Matthias Männich" <maennich@google.com>,
	kernel-team@android.com, "Kernel Team" <kernel-team@fb.com>,
	bpf <bpf@vger.kernel.org>
Subject: Re: [PATCH dwarves v3 2/5] btf_encoder: Do not use both structs and pointers for the same data
Date: Mon, 8 Feb 2021 14:23:31 -0800	[thread overview]
Message-ID: <CAEf4BzZP8hWkBUKi15j3E+D63n9269viQyCagsHpJFm=DOE4Gg@mail.gmail.com> (raw)
In-Reply-To: <20210205134221.2953163-3-gprocida@google.com>

On Fri, Feb 5, 2021 at 5:42 AM Giuliano Procida <gprocida@google.com> wrote:
>
> Many operations in the libelf API return a pointer to a user-provided
> struct (on success) or NULL (on failure).
>
> There are a couple of places in btf_elf__write where both structs and
> pointers to the same structs are used. Holding on to the pointers
> raises ownership and lifetime issues unncessarily and the code is

typo: unnecessarily

> cleaner with only a single access path for these data.
>
> The code now treats the returned pointers as booleans.
>
> Signed-off-by: Giuliano Procida <gprocida@google.com>
> ---

styling nits, but otherwise LGTM

Acked-by: Andrii Nakryiko <andrii@kernel.org>

>  libbtf.c | 14 ++++++--------
>  1 file changed, 6 insertions(+), 8 deletions(-)
>
> diff --git a/libbtf.c b/libbtf.c
> index 7bc49ba..ace8896 100644
> --- a/libbtf.c
> +++ b/libbtf.c
> @@ -698,8 +698,7 @@ int32_t btf_elf__add_datasec_type(struct btf_elf *btfe, const char *section_name
>
>  static int btf_elf__write(const char *filename, struct btf *btf)
>  {
> -       GElf_Shdr shdr_mem, *shdr;
> -       GElf_Ehdr ehdr_mem, *ehdr;
> +       GElf_Ehdr ehdr;
>         Elf_Data *btf_data = NULL;
>         Elf_Scn *scn = NULL;
>         Elf *elf = NULL;
> @@ -727,13 +726,12 @@ static int btf_elf__write(const char *filename, struct btf *btf)
>
>         elf_flagelf(elf, ELF_C_SET, ELF_F_DIRTY);
>
> -       ehdr = gelf_getehdr(elf, &ehdr_mem);
> -       if (ehdr == NULL) {
> +       if (!gelf_getehdr(elf, &ehdr)) {
>                 elf_error("elf_getehdr failed");
>                 goto out;
>         }
>
> -       switch (ehdr_mem.e_ident[EI_DATA]) {
> +       switch (ehdr.e_ident[EI_DATA]) {
>         case ELFDATA2LSB:
>                 btf__set_endianness(btf, BTF_LITTLE_ENDIAN);
>                 break;
> @@ -751,10 +749,10 @@ static int btf_elf__write(const char *filename, struct btf *btf)
>
>         elf_getshdrstrndx(elf, &strndx);
>         while ((scn = elf_nextscn(elf, scn)) != NULL) {
> -               shdr = gelf_getshdr(scn, &shdr_mem);
> -               if (shdr == NULL)
> +               GElf_Shdr shdr;

it's a good style to have an empty line between variable declaration
block and subsequent instructions


> +               if (!gelf_getshdr(scn, &shdr))
>                         continue;
> -               char *secname = elf_strptr(elf, strndx, shdr->sh_name);
> +               char *secname = elf_strptr(elf, strndx, shdr.sh_name);
>                 if (strcmp(secname, ".BTF") == 0) {
>                         btf_data = elf_getdata(scn, btf_data);
>                         break;
> --
> 2.30.0.478.g8a0d178c01-goog
>