devkitARM gcc 10 strncpy false positive warning

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LiquidFenrir
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Joined: Thu May 28, 2020 10:34 am

devkitARM gcc 10 strncpy false positive warning

Post by LiquidFenrir » Thu Jun 25, 2020 10:53 pm

Hello, I just found out a weird warning in the linking step of a c++17 3ds homebrew, that I haven't been able to replicate except on every compilation of that specific (unreleased for now) source:
I have a mInfo struct with a few char array members, the first of size 8, followed by sizes 16, 128 (mShortDesc), and 256 (mLongDesc).
When strncpy-ing from a std::string created from the utf16 to utf8 conversion of a SMDH's short description to the 128-long array (using 128 as the limit size), it warns saying:

Code: Select all

warning: 'strncpy' writing 128 bytes into a region of size 16 overflows the destination [-Wstringop-overflow=]
  425 |         strncpy(mInfo.mShortDesc, desc.c_str(), 0x80);
even though mInfo.mShortDesc is of size 128 (0x80).
And when doing the same for the SMDH's long description to the 256 long array (with 256 as the limit), it says:

Code: Select all

warning: 'strncpy' writing 256 bytes into a region of size 128 overflows the destination [-Wstringop-overflow=]
  427 |         strncpy(mInfo.mLongDesc, desc.c_str(), 0x100);
even though mInfo.mLongDescis of size 256 (0x100).

This looks like a compiler bug to me, because it seems to (wrongly) use the size of the previous member in the warning, so I thought it should be reported here (or maybe to gcc, but I'll let you do that if needed).

Thank you for the incredible libraries and software, I hope this can be fixed, or if it's not a bug and instead user error, know how not to do it.

WinterMute
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Re: devkitARM gcc 10 strncpy false positive warning

Post by WinterMute » Fri Jun 26, 2020 12:50 am

There's not really a lot we can do without a testcase.

If you can't get it to warn on a small testcase but only as part of a larger project then I'd be inclined towards saying that you're possibly misreading something in the original code.
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LiquidFenrir
Posts: 6
Joined: Thu May 28, 2020 10:34 am

Re: devkitARM gcc 10 strncpy false positive warning

Post by LiquidFenrir » Fri Jun 26, 2020 7:21 am

Oh actually by switching the keywords a bit and not being dead tired while searching, i found this very bug already on the gcc bugzilla... Not dkp's fault, sorry to have wasted your time. Seems like it's only a warning and doesnt impact the program, if I'm reading correctly.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87296

WinterMute
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Re: devkitARM gcc 10 strncpy false positive warning

Post by WinterMute » Fri Jun 26, 2020 10:22 am

It's not a waste of time to have this noted. Thanks for coming back to link the bugzilla report.
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