libcasper: Neuter false positive -Wuse-after-free warnings from GCC 13
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Authored by jhb on Nov 13 2023, 10:29 PM.

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emaste
imp

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rG1dcd99610434: libcasper: Neuter false positive -Wuse-after-free warnings from GCC 13
rGb7f7cc25c01a: libcasper: Neuter false positive -Wuse-after-free warnings from GCC 13

Summary

GCC 13 incorrectly thinks a call to free after a failed realloc is a
use after free.

lib/libcasper/services/cap_grp/cap_grp.c: In function 'group_resize':
lib/libcasper/services/cap_grp/cap_grp.c:65:17: error: pointer 'buf' may be used after 'realloc' [-Werror=use-after-free]

65 |                 free(buf);
   |                 ^~~~~~~~~

lib/libcasper/services/cap_grp/cap_grp.c:63:19: note: call to 'realloc' here

63 |         gbuffer = realloc(buf, gbufsize);
   |                   ^~~~~~~~~~~~~~~~~~~~~~

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rG FreeBSD src repository

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Build Status

Buildable 54400
Build 51290: arc lint + arc unit

Event Timeline

jhb created this revision.Nov 13 2023, 10:29 PM

Herald added a subscriber: imp. · View Herald TranscriptNov 13 2023, 10:29 PM

jhb requested review of this revision.Nov 13 2023, 10:29 PM

Harbormaster completed remote builds in B54400: Diff 130049.Nov 13 2023, 10:29 PM

jhb added a child revision: D42577: libgpio: Fix type mismatch for gpio_pin_[gs]et.Nov 13 2023, 10:29 PM

emaste accepted this revision.Nov 13 2023, 11:16 PM

This revision is now accepted and ready to land.Nov 13 2023, 11:16 PM

I thought this is becoming UB behavior...
But for our implementation it's well defined.

In D42576#971550, @imp wrote:

I thought this is becoming UB behavior...
But for our implementation it's well defined.

If realloc fails and returns NULL, the original pointer is always valid, that's how realloc is defined in C. There is some discussion about making realloc() with a new size of 0 as UB to permit varying implementations, but that isn't relevant here.

In D42576#971578, @jhb wrote:

In D42576#971550, @imp wrote:

I thought this is becoming UB behavior...
But for our implementation it's well defined.

If realloc fails and returns NULL, the original pointer is always valid, that's how realloc is defined in C. There is some discussion about making realloc() with a new size of 0 as UB to permit varying implementations, but that isn't relevant here.