write: fix reserving empty section error

[lzw3232]

if we copy a elf with a empty section and the sh_addralign(align_start) is not 0, it will return a unaligned offset.

[philipc]

Thanks for the PR. However, it does not look correct. Can you give an in depth explanation so I can better understand the problem? Are you running the elfcopy example? How does the unaligned offset cause an error?

[lzw3232]

Thanks for the PR. However, it does not look correct. Can you give an in depth explanation so I can better understand the problem? Are you running the elfcopy example? How does the unaligned offset cause an error?

for example, the reserved_len is 64. then the first section's len is 42, align_start is 16. the offset is 64. But next section's len is 0, align_start is 16. the offset should be 112, but the offset is 106.
you can run elfcopy with this elf https://github.com/lzw3232/test.

[philipc]

Ok, and does that offset of 106 causes problems? It does sound like something that needs fixing, but I want to understand the problem first.

[lzw3232]

Ok, and does that offset of 106 causes problems? It does sound like something that needs fixing, but I want to understand the problem first.

yes，the 106/16 is not a integer. so it is a unaligned offset.

[philipc]

Yes I understand what an unaligned offset is. What were you doing that caused you to notice that this is a problem?

I see that running elfcopy on it gives an error:

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `106`,
 right: `112`', crates/examples/src/bin/elfcopy.rs:506:21
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Is that all that you were trying to do, or are you trying to use this code in something else where the unaligned offset causes a problem?

[lzw3232]

Yes I understand what an unaligned offset is. What were you doing that caused you to notice that this is a problem?

I see that running elfcopy on it gives an error:

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `106`,
 right: `112`', crates/examples/src/bin/elfcopy.rs:506:21
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Is that all that you were trying to do, or are you trying to use this code in something else where the unaligned offset causes a problem?

I just want to copy a elf with a empty section. This function said that "Returns the aligned offset of the start of the range". But if len is 0, it doesn't call "util::align" just return "self.len". Then I think this is a problem.

[philipc]

Thanks!